I am using Thunderbird with Comcast and it works just fine.

Setting up email clients is pretty simple. Enter the correct server settings for your provider and it works. Intermittent problems are almost always on the providers end. They may update server setting and you have to make the needed changes but it does not go back and forth on a regular basis.

This email detector site seems to show certain Comcast areas having issues. http://downdetector.com/status/comcast-xfinity

If you go to the Thunderbird Help menu and click Troubleshooting Info, then Copy to Clipboard. Do not check the box to show account names. Come back here and paste that into a reply window.

I will compare your settings with mine to be sure.

Application Basics

   Name: Thunderbird
   Version: 52.7.0
   User Agent: Mozilla/5.0 (Windows NT 10.0; WOW64; rv:52.0) Gecko/20100101 Thunderbird/52.7.0
   Profile Folder: Open Folder

             (Local drive)
   Application Build ID: 20180322145026
   Enabled Plugins: about:plugins
   Build Configuration: about:buildconfig
   Memory Use: about:memory
   Profiles: about:profiles

 Mail and News Accounts
   account1:
     INCOMING: account1, , (pop3) mail.comcast.net:995, SSL, passwordCleartext
     OUTGOING: , smtp.comcast.net:465, SSL, passwordCleartext, true

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

   account3:
     INCOMING: account3, , (imap) imap-mail.outlook.com:993, SSL, passwordCleartext
     OUTGOING: , smtp-mail.outlook.com:587, alwaysSTARTTLS, passwordCleartext, true

 Crash Reports
   https://crash-stats.mozilla.com/report/index/bp-55b2c940-65fc-4de2-aba3-a18c32160927 (9/27/2016)

 Extensions
   Lightning, 5.4.7, false, {e2fda1a4-762b-4020-b5ad-a41df1933103}

 Important Modified Preferences

   Name: Value

     accessibility.lastLoadDate: 1506201590
     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: 52.7.0
     font.internaluseonly.changed: true
     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
     font.size.variable.x-western: 17
     mail.openMessageBehavior.version: 1
     mail.winsearch.firstRunDone: true
     mailnews.database.global.datastore.id: 380d18fa-c388-494b-a4be-05aa5ea361f
     mailnews.database.global.views.conversation.columns: {"threadCol":{"visible":true,"ordinal":"1"},"flaggedCol":{"visible":true,"ordinal":"3"},"attachmentCol":{"visible":false…
     media.gmp.storage.version.observed: 1
     network.cookie.prefsMigrated: true
     network.predictor.cleaned-up: true
     places.database.lastMaintenance: 1525191028
     places.history.expiration.transient_current_max_pages: 49967
     plugin.importedState: true
     security.sandbox.content.tempDirSuffix: {e4fc8cfa-eabd-4b58-b85b-fe1bb9b19906}

 Graphics

     GPU #1
     Description: Intel(R) HD Graphics 530
     Vendor ID: 0x8086
     Device ID: 0x191b
     RAM: Unknown
     Drivers: igdumdim64 igd10iumd64 igd10iumd64 igd12umd64 igdumdim32 igd10iumd32 igd10iumd32 igd12umd32
     Driver Version: 21.20.16.4542
     Driver Date: 10-24-2016

     Features
     Direct2D: false
     DirectWrite: true (10.0.16299.334)
     WebGL Renderer: Google Inc. -- ANGLE (Intel(R) HD Graphics 530 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
     3.28.6
     3.28.6

     NSS Util
     3.28.6
     3.28.6

     NSS SSL
     3.28.6
     3.28.6

     NSS S/MIME
     3.28.6
     3.28.6

Those are the same settings I have working with Comcast. Are you in any of the areas reporting Comcast issues on the site I linked to? If so did you read any of the comments?

No, I'm not in any of the areas reporting Comcast issues. I'm not having trouble with Comcast itself. I can access my email directly through the Xfinity site with no problem. I can also access my email on my phone with no problem. There are new messages on the Comcast server that I can access. The only issue I'm having is with downloading new messages on Thunderbird.

Web mail and phone apps do not use the same server as an email client does so that does not mean much.

If there's something that just changed within Thunderbird or Comcast, it worked. I was just able to download new messages. I changed nothing.

You may have had a corrupted message blocking downloads to TB, and this message was deleted by you via webmail or possibly during a manual or automatic cleanup of a spam folder.

Good thought but no, I didn't delete anything at all from my inbox. I had emptied the spam folder several times over the last 4 1/2 days with no luck. Whatever it was, Thunderbird is downloading fine for now.

Thunderbird is email client software running on your computer. If something changed in Thunderbird you would have had to do it. Glad it is back working. Hope it stays that way.