What is the latest crash ID, found in Help/More Troubleshooting, Crash Reports? It's a string beginning with bp-...

I created a new crash report (just by closing TB and waiting for several minutes while the task manager still showed Thunderbird-Inbox or something similar before the Mozilla crash reporter finally popped up):

Here is the latest ID: bp-8c1c8fd5-2568-4dd6-beb9-830d00211014

Older IDs are (all from within one week before I stopped sending crash reports until yesterday): bp-8787ff4a-8695-4523-a33e-9060e0210929 bp-fd5c664d-91d8-4397-aeeb-92e440210927 bp-a29d063b-cfd5-4601-bcfa-1325a0210826 bp-524f1211-21fb-4494-bece-6bc910210826 bp-265cfb7b-c0e6-4beb-aa60-111ba0210826 bp-b173ea49-cf8f-4521-a7d9-659c40210823 bp-f1ce1f9e-72de-4d09-90f8-ad59c0210823

I hope this helps to find the problem.

The crash reports refer to the Textmarker, Attach from Clipboard and XNote++ add-ons, and Dropbox. Check to see if any of the extensions are incompatible. Do you have some connection between TB and Dropbox, such as profile backups?

No connection to Dropbox - neither from Firefox nor Thunderbird (of course, I can open Dropbox URLs).

So, I should check removing or disabling the Textmarker, Attach from Clipboard and XNote++ add-ons ? Do you know when TB91 compatible versions of those will be available (even if not Mozilla is providing them) ?

PS: can you have a short look at my other question about opening detached attachments: https://support.mozilla.org/en-US/questions/1353549 Thanks for all support !

Ii think you have the latest versions for those add-ons, but one or more of them might not be compatible. Run in safe mode (hold Shift when launching TB) to test (add-ons are disabled in safe mode).