File menu > compact folders.

If you have no main menu bar press F10 to make it appear.

Thanks for your suggestionI tried it but it didn't work.

View/Layout/Folder Pane Columns, then click the icon to display the desired columns (see picture).

when I click "folder pane columns" on Layout all the folders disapear. Also it isn't clear to me which icon you're referring to.

You may have accidentally unchecked View/Layout/Folder Pane.

The icon I referred to is the highlighted one in the picture next to the Size column button.

No ,I didn't uncheck the Folder Pane Box. Also I hve nothing just above the Inbox word and its icon, so I don't know what you're referring to as the size column button. I have clicked on everything I see in that neighborhood and nothing happens.

In order for us to better investigate this issue you need to add a little more non-personal information.

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.

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) mail.q.com:995, SSL, passwordCleartext
     OUTGOING: , smtp.q.com:587, alwaysSTARTTLS, passwordCleartext, true

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

 Crash Reports
   https://crash-stats.mozilla.com/report/index/bp-731decd4-41c4-400f-acf4-42b570170422 (4/22/2017)

 Extensions
   Lightning, 5.4.3, true, {e2fda1a4-762b-4020-b5ad-a41df1933103}

 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: 13
     font.name.monospace.el: Consolas
     font.name.monospace.x-cyrillic: Consolas
     font.name.monospace.x-unicode: Consolas
     font.name.monospace.x-western: Consolas
     font.name.sans-serif.el: Calibri
     font.name.sans-serif.x-cyrillic: Calibri
     font.name.sans-serif.x-unicode: Calibri
     font.name.sans-serif.x-western: Calibri
     font.name.serif.el: 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.x-cyrillic: 14
     font.size.fixed.x-unicode: 14
     font.size.fixed.x-western: 14
     font.size.variable.el: 17
     font.size.variable.x-cyrillic: 17
     font.size.variable.x-unicode: 17
     gfx.crash-guard.glcontext.appVersion: 45.7.1
     gfx.crash-guard.glcontext.deviceID: 0x9830
     gfx.crash-guard.glcontext.driverVersion: 15.201.1101.0
     gfx.crash-guard.glcontext.gfx.driver-init.direct3d11-angle: true
     gfx.crash-guard.glcontext.gfx.driver-init.webgl-angle: true
     gfx.crash-guard.glcontext.gfx.driver-init.webgl-angle-force-d3d11: false
     gfx.crash-guard.glcontext.gfx.driver-init.webgl-angle-force-warp: false
     gfx.crash-guard.glcontext.gfx.driver-init.webgl-angle-try-d3d11: true
     gfx.crash-guard.status.glcontext: 2
     mail.openMessageBehavior.version: 1
     mail.winsearch.firstRunDone: true
     mailnews.database.global.datastore.id: 319ceb5c-4011-482c-ad63-c626de51961
     mailnews.database.global.views.conversation.columns: {"threadCol":{"visible":true,"ordinal":"1"},"flaggedCol":{"visible":true,"ordinal":"5"},"attachmentCol":{"visible":false…
     media.gmp.storage.version.observed: 1
     network.cookie.cookieBehavior: 2
     network.cookie.prefsMigrated: true
     network.predictor.cleaned-up: true
     places.database.lastMaintenance: 1506697760
     places.history.enabled: false
     places.history.expiration.transient_current_max_pages: 122334
     plugin.importedState: true
     print.printer_HP_ENVY_4520_series_(Network).print_bgcolor: false
     print.printer_HP_ENVY_4520_series_(Network).print_bgimages: false
     print.printer_HP_ENVY_4520_series_(Network).print_duplex: -437918235
     print.printer_HP_ENVY_4520_series_(Network).print_edge_bottom: 0
     print.printer_HP_ENVY_4520_series_(Network).print_edge_left: 0
     print.printer_HP_ENVY_4520_series_(Network).print_edge_right: 0
     print.printer_HP_ENVY_4520_series_(Network).print_edge_top: 0
     print.printer_HP_ENVY_4520_series_(Network).print_evenpages: true
     print.printer_HP_ENVY_4520_series_(Network).print_footercenter:
     print.printer_HP_ENVY_4520_series_(Network).print_footerleft: &PT
     print.printer_HP_ENVY_4520_series_(Network).print_footerright: &D
     print.printer_HP_ENVY_4520_series_(Network).print_headercenter:
     print.printer_HP_ENVY_4520_series_(Network).print_headerleft: &T
     print.printer_HP_ENVY_4520_series_(Network).print_headerright: &U
     print.printer_HP_ENVY_4520_series_(Network).print_in_color: true
     print.printer_HP_ENVY_4520_series_(Network).print_margin_bottom: 0.5
     print.printer_HP_ENVY_4520_series_(Network).print_margin_left: 0.5
     print.printer_HP_ENVY_4520_series_(Network).print_margin_right: 0.5
     print.printer_HP_ENVY_4520_series_(Network).print_margin_top: 0.5
     print.printer_HP_ENVY_4520_series_(Network).print_oddpages: true
     print.printer_HP_ENVY_4520_series_(Network).print_orientation: 0
     print.printer_HP_ENVY_4520_series_(Network).print_page_delay: 50
     print.printer_HP_ENVY_4520_series_(Network).print_paper_data: 1
     print.printer_HP_ENVY_4520_series_(Network).print_paper_height: 11.00
     print.printer_HP_ENVY_4520_series_(Network).print_paper_name:
     print.printer_HP_ENVY_4520_series_(Network).print_paper_size_type: 0
     print.printer_HP_ENVY_4520_series_(Network).print_paper_size_unit: 0
     print.printer_HP_ENVY_4520_series_(Network).print_paper_width: 8.50
     print.printer_HP_ENVY_4520_series_(Network).print_resolution: -437918235
     print.printer_HP_ENVY_4520_series_(Network).print_reversed: false
     print.printer_HP_ENVY_4520_series_(Network).print_scaling: 1.00
     print.printer_HP_ENVY_4520_series_(Network).print_shrink_to_fit: false
     print.printer_HP_ENVY_4520_series_(Network).print_to_file: false
     print.printer_HP_ENVY_4520_series_(Network).print_unwriteable_margin_bottom: 0
     print.printer_HP_ENVY_4520_series_(Network).print_unwriteable_margin_left: 0
     print.printer_HP_ENVY_4520_series_(Network).print_unwriteable_margin_right: 0
     print.printer_HP_ENVY_4520_series_(Network).print_unwriteable_margin_top: 0
     privacy.donottrackheader.enabled: true
     security.sandbox.content.tempDirSuffix: {538a7d9e-450a-4e5f-aeba-5a0853205854}

 Graphics

     GPU #1
     Description: AMD Radeon HD 8400 / R3 Series
     Vendor ID: 0x1002
     Device ID: 0x9830
     RAM: 512
     Drivers: aticfx64 aticfx64 aticfx64 amdxc64 aticfx32 aticfx32 aticfx32 amdxc32 atiumd64 atidxx64 atidxx64 atiumdag atidxx32 atidxx32 atiumdva atiumd6a atitmm64
     Driver Version: 15.201.1101.0
     Driver Date: 8-6-2015

     Features
     Direct2D: false
     DirectWrite: true (10.0.15063.413)
     WebGL Renderer: Google Inc. -- ANGLE (AMD Radeon HD 8400 / R3 Series Direct3D9Ex vs_3_0 ps_3_0) -- OpenGL ES 2.0 (ANGLE 2.1.0.2a250c8a0e15)

     AzureCanvasBackend: skia
     AzureCanvasAccelerated: 0
     AzureFallbackCanvasBackend: cairo
     AzureContentBackend: skia

 JavaScript

 Incremental GC: 1

 Accessibility

   Activated: 0
   Prevent Accessibility: 0

 Library Versions

     Expected minimum version
     Version in use

     NSPR
     4.13.1
     4.13.1

     NSS
     3.28.5
     3.28.5

     NSS Util
     3.28.5
     3.28.5

     NSS SSL
     3.28.5
     3.28.5

     NSS S/MIME
     3.28.5
     3.28.5