In Thunderbird 149.0b2 we see the following error:
Failed to load resource://gre/modules/PluralForm.sys.mjs quickfolders-quickMove.js:17:36
<anonymous> chrome://quickfolders/content/quickfolders-quickMove.js:17
<anonymous> jar:file:///I:/Thunderbird_Profiles/ct4akusa.Beta-Test/extensions/quickfolders@curious.be.xpi!/chrome/content/scripts/qf-messenger.js:20
_loadIntoWindow jar:file:///I:/Thunderbird_Profiles/ct4akusa.Beta-Test/extensions/quickfolders@curious.be.xpi!/chrome/content/api/WindowListener/implementation.js:729
AsyncFunctionNext self-hosted:780
this may be the cause for the toolbar not being displayed at all anymore.