Restart Thunderbird with add-ons disabled (Thunderbird Safe Mode). On the Help menu, click on "Restart with Add-ons Disabled". If Thunderbird works like normal, there is an Add-on or Theme interfering with normal operations. You will need to re-enable add-ons one at a time until you locate the offender.

Thank you for your quick response Matt. I've disabled all of my add-ons, including Lightening, shutdown T-bird, waited a few seconds, then restated.

I'm seeing some odd results. If I click on the Reply button to the email I got from Mozilla Support Forum with your message, the reply has my signature and the formatting shows your message. With other messages, when I click on the Reply button lose their formatting and have the ">" prepended to each line of text and there is no signature anywhere in the reply. If I take the same message and do Shift-Reply, the reply message is formatted correctly, but no signature is present.

Most confusing and annoying.

holding shit forces plain text or HTML message editing (the reverse of what you have set in preferences.)

I suggested safe mode. That is NOT the same as disabling add-ons and themes.

>> is the usual plain text message previous message quoting style HTML used vertical coloured lines.

Set the reply location to before the quoted text and the the signature location below it to :below my reply" Ensure the include signature in forwards is set.

Scroll to the bottom of the account list to "Outgoing server (SMTP) do you have more than one SMTP server configured for the relevant mail account?

I Think the issue is one where you are replying with two different SMTP servers and each has it's own configuration so the signature and text are handled according to the outgoing server used.

Please add the troubleshooting information to your post To find the Troubleshooting information:

• Open Help (or click on three-line-icon and select Help)
• Choose Troubleshooting Information
• Use the button Copy to clipboard to select all. Do not check box "Include account names"!
• Paste this in your post.

The behavior may be related to messages stored in local folders. I just noticed that the message from Mozilla Support is still in the in-box, while the other messages I'm replying to that don't get the signature and have ">" prepended are in local folders (plain text format?). This may be a coincidence. The messages in the local folders are formatted in the original form (HTML format?) when I use Shift-Reply, but get no signature.

Text based messages, such as ones from Cygwin, get a signature when I click "Reply" and are formatted without the ">" prepended (HTML format?), using the Shift-Reply the message has the prepended ">" characters and a signature (plain text?). It almost seems that if the original message is HTML, clicking on Reply will make the reply plain text, while if the message is plain text, Reply will use HTML formatting. Very strange.

Scroll to the bottom of the account list to "Outgoing server (SMTP) do you have more than one SMTP server configured for the relevant mail account? {snip} Please add the troubleshooting information to your post To find the Troubleshooting information: Open Help (or click on three-line-icon and select Help) Choose Troubleshooting Information Use the button Copy to clipboard to select all. Do not check box "Include account names"! Paste this in your post.

I will try again to get information and answers.

Application Basics

   Name: Thunderbird
   Version: 60.4.0
   Build ID: 20181217101353
   Update Channel: release
   User Agent: Mozilla/5.0 (Windows NT 10.0; WOW64; rv:60.0) Gecko/20100101 Thunderbird/60.4.0
   OS: Windows_NT 10.0
   Multiprocess Windows: 0/0 (Disabled)
   Web Content Processes:
   Stylo: content = true (enabled by default), chrome = true (enabled by default)
   Enterprise Policies:
   Google Key: Missing
   Mozilla Location Service Key: Missing
   Safe Mode: false

 Mail and News Accounts
   account1:
     INCOMING: account1, , (none) Local Folders, plain, passwordCleartext

   account2:
     INCOMING: account2, , (nntp) free.xsusenet.com:119, plain, passwordCleartext
     OUTGOING: , smtp.comcast.net:587, alwaysSTARTTLS, passwordCleartext, true

   account3:
     INCOMING: account3, , (imap) imap.comcast.net:993, SSL, passwordCleartext
     OUTGOING: , smtp.comcast.net:587, alwaysSTARTTLS, passwordCleartext, true

   account4:
     INCOMING: account4, , (pop3) inbound.att.net:995, SSL, passwordCleartext
     OUTGOING: , outbound.att.net:465, SSL, passwordCleartext, true

   account5:
     INCOMING: account5, , (imap) mail.ds.net:993, SSL, passwordCleartext
     OUTGOING: , mail.ds.net:465, SSL, passwordCleartext, true

   account6:
     INCOMING: account6, , (nntp) news.mozilla.org:119, plain, passwordCleartext
     OUTGOING: , smtp.comcast.net:587, alwaysSTARTTLS, passwordCleartext, true

 Crash Reports for the Last 3 Days
 All Crash Reports
 Extensions
     Name
     Version
     Enabled
     ID

       Advanced Unread Folders
       2.4
       true
       fp-unread@mozilla.kewis.ch

       Attach from Clipboard
       1.0.2
       true
       clipboard@ganss.org

       Auto Resize Image
       1.1.0
       true
       AutoResizeImage@gmail.com

       Auto Save Drafts Folders
       0.3.0
       true
       asdf@d.lech

       Awesome Auto Archive
       0.8
       true
       awsomeautoarchive@opera.wang

       Duplicate Contacts Manager
       1.1
       true
       duplicateContactsManager@ddvo.net

       Emoji
       1.5.8
       true
       emoji@ganss.org

       Enigmail
       2.0.9
       true
       {847b3a00-7ab1-11d4-8f02-006008948af5}

       Expression Search / Google Mail UI
       1.3
       true
       {03EF8A6E-C972-488f-92FA-98ABC2C9F8B9}

       Forward
       0.19
       true
       forward@mozdev.org

       Gantt View
       1.0.0.60
       true
       {1510928E-A4FA-43C2-A9AF-99E521B90BC5}

       Grammar Checker
       0.8.4
       true
       grammarchecker@codedgers.com

       Hyperactive
       2018.08.13
       true
       hyperactive@urihart.addons.mozilla.org

       ImportExportTools
       3.3.0
       true
       {3ed8cc52-86fc-4613-9026-c1ef969da4c3}

       Lightning
       6.2.4
       true
       {e2fda1a4-762b-4020-b5ad-a41df1933103}

       Lightning Invitation Notifier (LIN)
       1.4
       true
       lightninginvitationnotifier@markus.pullmann

       LookOut (fix version)
       1.2.23
       true
       lookout@s3_fix_version

       Mail Merge
       4.12.0
       true
       mailmerge@example.net

       Mail Redirect
       0.10.1
       true
       {CC3C233D-6668-41bc-AAEB-F3A1D1D594F5}

       Mailbox Alert
       0.18.0
       true
       {9c21158b-2c76-4d0a-980a-c51fc9cefaa7}

       PrintingTools
       1.3.0
       true
       {5e9999c2-ba1d-44b6-bcee-5b30ce37d3b0}

       Remind It
       2.1a
       true
       RemindIt@o.marce.free.fr

       Remove Duplicate Messages (Alternate)
       0.3.17b1
       true
       {a300a000-5e21-4ee0-a115-9ec8f4eaa92b}

       Reply As Original Recipient
       1.1
       true
       replyasoriginalrecipient@qiqitori.com

       Reply to Multiple Messages
       1.0.5
       true
       reply-to-multiple-messages@kamens.us

       Security Settings from Address Book
       2.0.1
       true
       securitysettingsfromaddressbook@gwu.at

       Send Later
       6.4.6
       true
       sendlater3@kamens.us

       Send Later Button
       1.3.1
       true
       sendlaterbutton@thunderbird-mail.de

       Sensitivity Header
       0.2
       true
       sensitivity_header_management@ff_bob.bob

       Stop Ignoring Reply:To
       1.2
       true
       stopignoringreplyto@gelvins.com

       ThunderHTMLedit
       2.0.0
       true
       jorgk@thunderHTMLedit

       Xpunge
       0.7.1
       true
       {786abda0-fd14-d247-bf69-38b2fc18491b}

 Security Software
   Type: Name
     Antivirus: Avast Antivirus
     Antispyware: Avast Antivirus
     Firewall: Avast Antivirus

Graphics

     Features
     Compositing: Direct3D 11 (Advanced Layers)
     Asynchronous Pan/Zoom: none
     WebGL 1 Driver WSI Info: EGL_VENDOR: Google Inc. (adapter LUID: 000000000000c2e0)

EGL_VERSION: 1.4 (ANGLE 2.1.0.f474fbb607e0) EGL_EXTENSIONS: EGL_EXT_create_context_robustness EGL_ANGLE_d3d_share_handle_client_buffer EGL_ANGLE_d3d_texture_client_buffer EGL_ANGLE_surface_d3d_texture_2d_share_handle EGL_ANGLE_query_surface_pointer EGL_ANGLE_window_fixed_size EGL_ANGLE_keyed_mutex EGL_ANGLE_surface_orientation EGL_ANGLE_direct_composition EGL_NV_post_sub_buffer EGL_KHR_create_context EGL_EXT_device_query EGL_KHR_image EGL_KHR_image_base EGL_KHR_gl_texture_2D_image EGL_KHR_gl_texture_cubemap_image EGL_KHR_gl_renderbuffer_image EGL_KHR_get_all_proc_addresses EGL_KHR_stream EGL_KHR_stream_consumer_gltexture EGL_NV_stream_consumer_gltexture_yuv EGL_ANGLE_flexible_surface_compatibility EGL_ANGLE_stream_producer_d3d_texture EGL_ANGLE_create_context_webgl_compatibility EGL_CHROMIUM_create_context_bind_generates_resource EGL_CHROMIUM_sync_control EGL_EXT_pixel_format_float EGL_KHR_surfaceless_context EGL_ANGLE_display_texture_share_group EGL_ANGLE_create_context_client_arrays EGL_ANGLE_program_cache_control EGL_ANGLE_robust_resource_initialization EGL_EXTENSIONS(nullptr): EGL_EXT_client_extensions EGL_EXT_platform_base EGL_EXT_platform_device EGL_ANGLE_platform_angle EGL_ANGLE_platform_angle_d3d EGL_ANGLE_device_creation EGL_ANGLE_device_creation_d3d11 EGL_ANGLE_experimental_present_path EGL_KHR_client_get_all_proc_addresses

     WebGL 1 Driver Renderer: Google Inc. -- ANGLE (Intel(R) HD Graphics Direct3D11 vs_5_0 ps_5_0)
     WebGL 1 Driver Version: OpenGL ES 2.0 (ANGLE 2.1.0.f474fbb607e0)
     WebGL 1 Driver Extensions: GL_ANGLE_client_arrays GL_ANGLE_depth_texture GL_ANGLE_framebuffer_blit GL_ANGLE_framebuffer_multisample GL_ANGLE_instanced_arrays GL_ANGLE_lossy_etc_decode GL_ANGLE_pack_reverse_row_order GL_ANGLE_program_cache_control GL_ANGLE_request_extension GL_ANGLE_robust_client_memory GL_ANGLE_texture_compression_dxt3 GL_ANGLE_texture_compression_dxt5 GL_ANGLE_texture_usage GL_ANGLE_translated_shader_source GL_CHROMIUM_bind_generates_resource GL_CHROMIUM_bind_uniform_location GL_CHROMIUM_color_buffer_float_rgb GL_CHROMIUM_color_buffer_float_rgba GL_CHROMIUM_copy_compressed_texture GL_CHROMIUM_copy_texture GL_CHROMIUM_sync_query GL_EXT_blend_minmax GL_EXT_color_buffer_half_float GL_EXT_debug_marker GL_EXT_discard_framebuffer GL_EXT_disjoint_timer_query GL_EXT_draw_buffers GL_EXT_frag_depth GL_EXT_map_buffer_range GL_EXT_occlusion_query_boolean GL_EXT_read_format_bgra GL_EXT_robustness GL_EXT_sRGB GL_EXT_shader_texture_lod GL_EXT_texture_compression_dxt1 GL_EXT_texture_compression_s3tc_srgb GL_EXT_texture_filter_anisotropic GL_EXT_texture_format_BGRA8888 GL_EXT_texture_rg GL_EXT_texture_storage GL_EXT_unpack_subimage GL_KHR_debug GL_NV_EGL_stream_consumer_external GL_NV_fence GL_NV_pack_subimage GL_NV_pixel_buffer_object GL_OES_EGL_image GL_OES_EGL_image_external GL_OES_compressed_ETC1_RGB8_texture GL_OES_depth32 GL_OES_element_index_uint GL_OES_get_program_binary GL_OES_mapbuffer GL_OES_packed_depth_stencil GL_OES_rgb8_rgba8 GL_OES_standard_derivatives GL_OES_surfaceless_context GL_OES_texture_float GL_OES_texture_float_linear GL_OES_texture_half_float GL_OES_texture_half_float_linear GL_OES_texture_npot GL_OES_vertex_array_object
     WebGL 1 Extensions: ANGLE_instanced_arrays EXT_blend_minmax EXT_color_buffer_half_float EXT_frag_depth EXT_sRGB EXT_shader_texture_lod EXT_texture_filter_anisotropic EXT_disjoint_timer_query OES_element_index_uint OES_standard_derivatives OES_texture_float OES_texture_float_linear OES_texture_half_float OES_texture_half_float_linear OES_vertex_array_object WEBGL_color_buffer_float WEBGL_compressed_texture_s3tc WEBGL_compressed_texture_s3tc_srgb WEBGL_debug_renderer_info WEBGL_debug_shaders WEBGL_depth_texture WEBGL_draw_buffers WEBGL_lose_context
     WebGL 2 Driver WSI Info: EGL_VENDOR: Google Inc. (adapter LUID: 000000000000c2e0)

EGL_VERSION: 1.4 (ANGLE 2.1.0.f474fbb607e0) EGL_EXTENSIONS: EGL_EXT_create_context_robustness EGL_ANGLE_d3d_share_handle_client_buffer EGL_ANGLE_d3d_texture_client_buffer EGL_ANGLE_surface_d3d_texture_2d_share_handle EGL_ANGLE_query_surface_pointer EGL_ANGLE_window_fixed_size EGL_ANGLE_keyed_mutex EGL_ANGLE_surface_orientation EGL_ANGLE_direct_composition EGL_NV_post_sub_buffer EGL_KHR_create_context EGL_EXT_device_query EGL_KHR_image EGL_KHR_image_base EGL_KHR_gl_texture_2D_image EGL_KHR_gl_texture_cubemap_image EGL_KHR_gl_renderbuffer_image EGL_KHR_get_all_proc_addresses EGL_KHR_stream EGL_KHR_stream_consumer_gltexture EGL_NV_stream_consumer_gltexture_yuv EGL_ANGLE_flexible_surface_compatibility EGL_ANGLE_stream_producer_d3d_texture EGL_ANGLE_create_context_webgl_compatibility EGL_CHROMIUM_create_context_bind_generates_resource EGL_CHROMIUM_sync_control EGL_EXT_pixel_format_float EGL_KHR_surfaceless_context EGL_ANGLE_display_texture_share_group EGL_ANGLE_create_context_client_arrays EGL_ANGLE_program_cache_control EGL_ANGLE_robust_resource_initialization EGL_EXTENSIONS(nullptr): EGL_EXT_client_extensions EGL_EXT_platform_base EGL_EXT_platform_device EGL_ANGLE_plat

Sorry, but there doesn't seem to be a way to attach this information as a file.

P.S. Is there any way to always force HTML formatting in all replies?