Some add-ons have caused this in the past. Is it still greyed out in safe/troubleshoot mode? Help/Troubleshoot Mode.

Thanks. Still greyed out in troubleshoot mode. I have never knowingly installed a Thunderbird add-on.

Does Ctrl+Enter or File/Send Now work to send? Any difference if you have a single To address, no Reply-to or Bcc address?

File->Send/Send Later also grayed out.

Deleting address in Bcc field makes Send active for a NEW message. This does not work for a reply.

I've now downgraded to version 78.8.1. Send works, but MY ADDRESS BOOK IS GONE. Did Version 91 change addressbook format? Is there a recovery for this?

You can try renaming abook.sqlite* to abook.sqlite*.bak, renaming abook.mab.bak to abook.mab, with TB closed, then restart TB and see if Personal Address Book is recovered.

Going back and forth with different TB versions using the same profile is not recommended. It used to work, but not since the last few versions.

I have no files with names containing "abook" anywhere, on either of my Linux systems. I looked in backups from before today's upgrade to version 91 of T-bird, and there are none their, either.

FWIW, my Mozilla/Firefox/Thunderbird profiles go back to Netscape days, with directories having been restored through multiple system changes over the past 20+ years.

Every TB profile folder has an abook (Personal Address Book) and history (Collected Addresses), either a .mab file or the newer .sqlite format. They are the default address books and cannot be deleted.

If you don't have any backups in LDIF or csv format, it is possible to import .mab files into TB 91 from Tools/Import.

Thanks, again. I did locate an 'abook.mab' file in a SeaMonkey profile directory, but it appears to have no content, and hasn't been modified in seven years. Otherwise, I have no files in the ~.thunderbird profile directory whose names contain the string "abook". The "backups" I mentioned are full filesystem backups; viewing their contents shows no such files either, so it doesn't appear they were lost today with the upgrade to version 91, yet the address book worked yesterday with version 78. (Mentioning again, I'm on Fedora Linux 34.)

(I'm thinking I need to start a new thread, reporting on several issues I've now encountered with version 91, not limited to this one...)