Troubleshooting info:

 Application Basics
   Name: Thunderbird
   Version: 52.3.0
   User Agent: Mozilla/5.0 (Windows NT 10.0; WOW64; rv:52.0) Gecko/20100101 Thunderbird/52.3.0
   Profile Folder: Open Folder

             (Local drive)
   Application Build ID: 20170815040357
   Enabled Plugins: about:plugins
   Build Configuration: about:buildconfig
   Memory Use: about:memory
   Profiles: about:profiles

 Mail and News Accounts
   account1:
     INCOMING: account1, , (pop3) pop.earthlink.net:110, plain, passwordEncrypted
     OUTGOING: , smtpauth.earthlink.net:587, alwaysSTARTTLS, passwordEncrypted, true

   account2:
     INCOMING: account2, , (none) Local Folders, plain, passwordCleartext

   account3:
     INCOMING: account3, , (pop3) pop.earthlink.net:110, plain, passwordEncrypted
     OUTGOING: , smtpauth.earthlink.net:587, alwaysSTARTTLS, passwordEncrypted, true

   account5:
     INCOMING: account5, , (pop3) pop.earthlink.net:110, plain, passwordEncrypted
     OUTGOING: , smtpauth.earthlink.net:587, alwaysSTARTTLS, passwordEncrypted, true

 Crash Reports

 Extensions

 Important Modified Preferences

   Name: Value

     browser.cache.disk.capacity: 358400
     browser.cache.disk.filesystem_reported: 1
     browser.cache.disk.smart_size_cached_value: 358400
     browser.cache.disk.smart_size.first_run: false
     browser.cache.disk.smart_size.use_old_max: false
     dom.apps.reset-permissions: true
     extensions.lastAppVersion: 52.3.0
     font.internaluseonly.changed: true
     font.minimum-size.x-western: 12
     font.name.monospace.el: Consolas
     font.name.monospace.tr: Consolas
     font.name.monospace.x-baltic: Consolas
     font.name.monospace.x-central-euro: Consolas
     font.name.monospace.x-cyrillic: Consolas
     font.name.monospace.x-unicode: Consolas
     font.name.monospace.x-western: Courier
     font.name.sans-serif.el: Calibri
     font.name.sans-serif.tr: Calibri
     font.name.sans-serif.x-baltic: Calibri
     font.name.sans-serif.x-central-euro: Calibri
     font.name.sans-serif.x-cyrillic: Calibri
     font.name.sans-serif.x-unicode: Calibri
     font.name.sans-serif.x-western: Verdana
     font.name.serif.el: Cambria
     font.name.serif.tr: Cambria
     font.name.serif.x-baltic: Cambria
     font.name.serif.x-central-euro: Cambria
     font.name.serif.x-cyrillic: Cambria
     font.name.serif.x-unicode: Cambria
     font.name.serif.x-western: Cambria
     font.size.fixed.el: 14
     font.size.fixed.tr: 14
     font.size.fixed.x-baltic: 14
     font.size.fixed.x-central-euro: 14
     font.size.fixed.x-cyrillic: 14
     font.size.fixed.x-unicode: 14
     font.size.fixed.x-western: 12
     font.size.variable.el: 17
     font.size.variable.tr: 17
     font.size.variable.x-baltic: 17
     font.size.variable.x-central-euro: 17
     font.size.variable.x-cyrillic: 17
     font.size.variable.x-unicode: 17
     font.size.variable.x-western: 12
     gfx.direct3d.last_used_feature_level_idx: 1
     mail.openMessageBehavior.version: 1
     mail.winsearch.firstRunDone: true
     mailnews.database.global.datastore.id: 2044cd77-a5cf-4590-b6f3-1cc5d594bab
     mailnews.database.global.views.conversation.columns: {"threadCol":{"visible":true,"ordinal":"19"},"flaggedCol":{"visible":true,"ordinal":"5"},"attachmentCol":{"visible":fals…
     mailnews.database.global.views.global.columns: {"threadCol":{"visible":true,"ordinal":"19"},"flaggedCol":{"visible":true,"ordinal":"5"},"attachmentCol":{"visible":fals…
     media.gmp.storage.version.observed: 1
     network.cookie.prefsMigrated: true
     network.predictor.cleaned-up: true
     places.database.lastMaintenance: 1507979497
     places.history.expiration.transient_current_max_pages: 49262
     plugin.importedState: true
     plugin.state.npbestbuypcappdetector: 0
     print.printer_Brother_MFC-J475DW_Printer.print_bgcolor: false
     print.printer_Brother_MFC-J475DW_Printer.print_bgimages: false
     print.printer_Brother_MFC-J475DW_Printer.print_colorspace:
     print.printer_Brother_MFC-J475DW_Printer.print_command:
     print.printer_Brother_MFC-J475DW_Printer.print_downloadfonts: false
     print.printer_Brother_MFC-J475DW_Printer.print_duplex: 1515870810
     print.printer_Brother_MFC-J475DW_Printer.print_edge_bottom: 0
     print.printer_Brother_MFC-J475DW_Printer.print_edge_left: 0
     print.printer_Brother_MFC-J475DW_Printer.print_edge_right: 0
     print.printer_Brother_MFC-J475DW_Printer.print_edge_top: 0
     print.printer_Brother_MFC-J475DW_Printer.print_evenpages: true
     print.printer_Brother_MFC-J475DW_Printer.print_footercenter:
     print.printer_Brother_MFC-J475DW_Printer.print_footerleft: &PT
     print.printer_Brother_MFC-J475DW_Printer.print_footerright: &D
     print.printer_Brother_MFC-J475DW_Printer.print_headercenter:
     print.printer_Brother_MFC-J475DW_Printer.print_headerleft: &T
     print.printer_Brother_MFC-J475DW_Printer.print_headerright: &U
     print.printer_Brother_MFC-J475DW_Printer.print_in_color: true
     print.printer_Brother_MFC-J475DW_Printer.print_margin_bottom: 0.5
     print.printer_Brother_MFC-J475DW_Printer.print_margin_left: 0.5
     print.printer_Brother_MFC-J475DW_Printer.print_margin_right: 0.5
     print.printer_Brother_MFC-J475DW_Printer.print_margin_top: 0.5
     print.printer_Brother_MFC-J475DW_Printer.print_oddpages: true
     print.printer_Brother_MFC-J475DW_Printer.print_orientation: 0
     print.printer_Brother_MFC-J475DW_Printer.print_page_delay: 50
     print.printer_Brother_MFC-J475DW_Printer.print_paper_data: 1
     print.printer_Brother_MFC-J475DW_Printer.print_paper_height: 11.00
     print.printer_Brother_MFC-J475DW_Printer.print_paper_name:
     print.printer_Brother_MFC-J475DW_Printer.print_paper_size_type: 0
     print.printer_Brother_MFC-J475DW_Printer.print_paper_size_unit: 0
     print.printer_Brother_MFC-J475DW_Printer.print_paper_width: 8.50
     print.printer_Brother_MFC-J475DW_Printer.print_plex_name:
     print.printer_Brother_MFC-J475DW_Printer.print_resolution: 1515870810
     print.printer_Brother_MFC-J475DW_Printer.print_resolution_name:
     print.printer_Brother_MFC-J475DW_Printer.print_reversed: false
     print.printer_Brother_MFC-J475DW_Printer.print_scaling: 1.00
     print.printer_Brother_MFC-J475DW_Printer.print_shrink_to_fit: true
     print.printer_Brother_MFC-J475DW_Printer.print_to_file: false
     print.printer_Brother_MFC-J475DW_Printer.print_unwriteable_margin_bottom: 0
     print.printer_Brother_MFC-J475DW_Printer.print_unwriteable_margin_left: 0
     print.printer_Brother_MFC-J475DW_Printer.print_unwriteable_margin_right: 0
     print.printer_Brother_MFC-J475DW_Printer.print_unwriteable_margin_top: 0
     print.printer_HP_Photosmart_D110_series.print_bgcolor: false
     print.printer_HP_Photosmart_D110_series.print_bgimages: false
     print.printer_HP_Photosmart_D110_series.print_colorspace:
     print.printer_HP_Photosmart_D110_series.print_command:
     print.printer_HP_Photosmart_D110_series.print_downloadfonts: false
     print.printer_HP_Photosmart_D110_series.print_duplex: 0
     print.printer_HP_Photosmart_D110_series.print_edge_bottom: 0
     print.printer_HP_Photosmart_D110_series.print_edge_left: 0
     print.printer_HP_Photosmart_D110_series.print_edge_right: 0
     print.printer_HP_Photosmart_D110_series.print_edge_top: 0
     print.printer_HP_Photosmart_D110_series.print_evenpages: true
     print.printer_HP_Photosmart_D110_series.print_footercenter:
     print.printer_HP_Photosmart_D110_series.print_footerleft: &PT
     print.printer_HP_Photosmart_D110_series.print_footerright: &D
     print.printer_HP_Photosmart_D110_series.print_headercenter:
     print.printer_HP_Photosmart_D110_series.print_headerleft: &T
     print.printer_HP_Photosmart_D110_series.print_headerright: &U
     print.printer_HP_Photosmart_D110_series.print_in_color: true
     print.printer_HP_Photosmart_D110_series.print_margin_bottom: 0.5
     print.printer_HP_Photosmart_D110_series.print_margin_left: 0.5
     print.printer_HP_Photosmart_D110_series.print_margin_right: 0.5
     print.printer_HP_Photosmart_D110_series.print_margin_top: 0.5
     print.printer_HP_Photosmart_D110_series.print_oddpages: true
     print.printer_HP_Photosmart_D110_series.print_orientation: 0
     print.printer_HP_Photosmart_D110_series.print_page_delay: 50
     print.printer_HP_Photosmart_D110_series.print_paper_data: 1
     print.printer_HP_Photosmart_D110_series.print_paper_height: 11.00
     print.printer_HP_Photosmart_D110_series.print_paper_name:
     print.printer_HP_Photosmart_D110_series.print_paper_size_type: 0
     print.printer_HP_Photosmart_D110_series.print_paper_size_unit: 0
     print.printer_HP_Photosmart_D110_series.print_paper_width: 8.50
     print.printer_HP_Photosmart_D110_series.print_plex_name:
     print.printer_HP_Photosmart_D110_series.print_resolution: 0
     print.printer_HP_Photosmart_D110_series.print_resolution_name:
     print.printer_HP_Photosmart_D110_series.print_reversed: false
     print.printer_HP_Photosmart_D110_series.print_scaling: 1.00
     print.printer_HP_Photosmart_D110_series.print_shrink_to_fit: true
     print.printer_HP_Photosmart_D110_series.print_to_file: false
     print.printer_HP_Photosmart_D110_series.print_unwriteable_margin_bottom: 0
     print.printer_HP_Photosmart_D110_series.print_unwriteable_margin_left: 0
     print.printer_HP_Photosmart_D110_series.print_unwriteable_margin_right: 0
     print.printer_HP_Photosmart_D110_series.print_unwriteable_margin_top: 0
     security.sandbox.content.tempDirSuffix: {611e9f1b-fe04-441a-b06c-47f6256fb795}

I've done the "Restart Now" option for Lightning several times and it seems to have no effect - still shows as disabled and still shows the option to "Restart Now".

From your picture, it looks like you have the Calendar open in a tab, which might disable Lightning on restart. Close the tab, restart TB, and see if you can enable Lightning.

Didn't have the calendar open, but did the restart as you asked anyway. No change. See attached screenshot. Thanks!

When you reinstalled Lightning, did you first Remove it from the Add-ons Manager, restart TB, and then install it again?

I've never seen a remove option in Add-ons Manager. The only thing close is the Undo option. Where would I find the remove option?

I've attached a picture of my Tools/Add-ons/Extensions tab. When you click 'More' for any add-on, you should still see buttons for Options, Disable/Enable and Remove, which are absent in your picture. Which makes me think you have a damaged or incomplete TB installation. A reinstall of TB with the full installer will not affect your profile folder. Manually removing the Lightning add-on involves deleting the {e2fda1a4-762b-4020-b5ad-a41df1933103} folder in the extensions subfolder of the profile folder (Help/Troubleshooting Information, click Open Folder, close TB before editing the profile folder).

Ok. Shut down TB, deleted the {e2fda1a4-762b-4020-b5ad-a41df1933103} folder, re-installed TB, restarted TB and attempted to install Lightning again - same exact results. See attached screenshot.

Issue resolved. I decided to shut down TB and then reboot the computer (I'd done this several times previously without success, but why not?). After the reboot, I started TB again and lo and behold, Lightning is back working with everything intact. I am at a loss to explain how and why that worked, but I'm not going to look a gift horse in the mouth.

Thank you for all of your efforts today in trying to help me resolve this. I have to think that at least one of the steps you suggested contributed to the solution and I truly appreciate the assistance. Have a great rest of your day!