https://support.mozilla.org/kb/cannot-receive-messages

Thunderbird doesn't care about your modem as long as the Internet connection is working.

I was wondering if there was a setting in Thunderbird that I had to change like a new MAC address or something.

looking up the modem model, it appears that it is a bridge, so unless you old modem did some firewall port mapping this does not then it should not be relevant.

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
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Application Basics

   Name: Thunderbird
   Version: 45.3.0
   User Agent: Mozilla/5.0 (Windows NT 6.3; WOW64; rv:45.0) Gecko/20100101 Thunderbird/45.3.0
   Profile Folder: Show Folder

             (Local drive)
   Application Build ID: 20160825102941
   Enabled Plugins: about:plugins
   Build Configuration: about:buildconfig
   Memory Use: about:memory

 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

 Crash Reports

 Extensions
   Lightning, 4.7.3, true, {e2fda1a4-762b-4020-b5ad-a41df1933103}

 Important Modified Preferences

   Name: Value

     accessibility.lastLoadDate: 1474759224
     accessibility.typeaheadfind.flashBar: 0
     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: 45.3.0
     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: Consolas
     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: Calibri
     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: 14
     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: 17
     gfx.crash-guard.glcontext.appVersion: 45.2.0
     gfx.crash-guard.glcontext.deviceID: 0x0116
     gfx.crash-guard.glcontext.driverVersion: 9.17.10.3347
     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
     gfx.direct3d.last_used_feature_level_idx: 0
     mail.openMessageBehavior.version: 1
     mail.winsearch.firstRunDone: true
     mailnews.database.global.datastore.id: e403cdd1-d1b9-4460-9905-0dbeaf05c81
     network.cookie.prefsMigrated: true
     network.predictor.cleaned-up: true
     places.database.lastMaintenance: 1474002152
     places.history.expiration.transient_current_max_pages: 104323
     plugin.importedState: true
     privacy.donottrackheader.enabled: true

 Graphics

     Adapter Description: Intel(R) HD Graphics 3000
     Vendor ID: 0x8086
     Device ID: 0x0116
     Adapter RAM: Unknown
     Adapter Drivers: igdumd64 igd10umd64 igd10umd64 igdumd32 igd10umd32 igd10umd32
     Driver Version: 9.17.10.3347
     Driver Date: 1-29-2014
     Direct2D Enabled: false
     DirectWrite Enabled: false (6.3.9600.18123)
     ClearType Parameters: ClearType parameters not found
     WebGL Renderer: Google Inc. -- ANGLE (Intel(R) HD Graphics 3000 Direct3D11 vs_4_1 ps_4_1) -- OpenGL ES 2.0 (ANGLE 2.1.0.316930d51ea9)
     GPU Accelerated Windows: 0

     AzureCanvasBackend: skia
     AzureSkiaAccelerated: 0
     AzureFallbackCanvasBackend: cairo
     AzureContentBackend: cairo

 JavaScript

 Incremental GC: 1

 Accessibility

   Activated: 1
   Prevent Accessibility: 0

 Library Versions

     Expected minimum version
     Version in use

     NSPR
     4.12
     4.12

     NSS
     3.21.1 Basic ECC
     3.21.1 Basic ECC

     NSS Util
     3.21.1
     3.21.1

     NSS SSL
     3.21.1 Basic ECC
     3.21.1 Basic ECC

     NSS S/MIME
     3.21.1 Basic ECC
     3.21.1 Basic ECC

Try this: Ensure router (Netgear N600) is powered & connected to modem (Arris6141) then cycle power on Arris6141 to restart modem.

But if you are getting a password error, then this would assume your computer can access the internet and so there should not be anything wrong with the modem and router.

The pop mail account info looks correct.

Suggest you check your 'User Name' is your full email address and password does not have any spelling typos.

Your User Name and Password are stored here: 'Menu icon' > 'Options' > 'Options' > 'Security' > 'Passwords' tab click on 'Saved Passwords' button click on 'Show Passwords' button

Right click on line to get popup which allows editing. cllick on all OK's.

Just had a thought..... log on the webmail comcast account and check to see if the POP option is selected. Maybe the default is IMAP and you have to select the POP if you want it.

Spent 1.5 hours on chat with Comcast. They finally told me to change the primary comcast password then change the outgoing mail settings in Thunderbird and it finally accepted the new password. Thank you for all the suggestions.