Is your "local" Thunderbird data stored on a network device? Does the problem change if you do Help > Troubleshoot Mode?

Thank you, but all is on drive C:, and Troubleshoot mode did not help.

Please check Error Console (ctrl+shift+J) for errors and post same here.

1748575453066 addons.xpi WARN Checking C:\Program Files (x86)\Mozilla Thunderbird\distribution\extensions for addons UnknownCollectionError: Unknown Collection "thunderbird/moz-essential-domain-fallbacks"

   UnknownCollectionError resource://services-settings/RemoteSettingsClient.sys.mjs:189
   sync resource://services-settings/RemoteSettingsClient.sys.mjs:635

EssentialDomainsRemoteSettings.sys.mjs:110:15 UnknownCollectionError: Unknown Collection "thunderbird/url-parser-default-unknown-schemes-interventions"

   UnknownCollectionError resource://services-settings/RemoteSettingsClient.sys.mjs:189
   sync resource://services-settings/RemoteSettingsClient.sys.mjs:635

SimpleURIUnknownSchemesRemoteObserver.sys.mjs:114:15 services.settings: Remote Settings startup changesets bundle could not be extracted (TypeError: serverInfo.capabilities.attachments is undefined) remote-settings.sys.mjs:227 services.settings: UnknownCollectionError: Unknown Collection "thunderbird/url-classifier-exceptions"

   UnknownCollectionError resource://services-settings/RemoteSettingsClient.sys.mjs:189
   sync resource://services-settings/RemoteSettingsClient.sys.mjs:635

RemoteSettingsClient.sys.mjs:555 WebGL context was lost. 2 Troubleshoot.sys.mjs:727:17 Uncaught NS_ERROR_ILLEGAL_VALUE: Component returned failure code: 0x80070057 (NS_ERROR_ILLEGAL_VALUE) [nsIMsgMessageService.messageURIToMsgHdr]

   displayMessage chrome://messenger/content/aboutMessage.js:269
   openTab chrome://messenger/content/mailTabs.js:305
   _cloneTemplate chrome://messenger/content/mailTabs.js:52

5 aboutMessage.js:269

​

All suddenly appeared. You're a magician! Thanks, Wayne.

DOes https://bugzilla.mozilla.org/show_bug.cgi?id=1963630 describe your issue?

All I lost was the tabs, and I can live with that.

Changed to dark mode and even the tabs reappeared. Back to light mode and they held firm. I'm 100% now.

Thanks

COOL. And very strange. Thanks for hanging in there.