I managed to find two examples of the same thing happening to other people (albeit on other operating systems, Dad in on Windows 10). The first is this: https://www.spinics.net/linux/fedora/fe ... 09032.html
I'm not sure what the mailspool/movemail thing is, but the rest of this accurately describe my father's problem, including having it set to BCC himself on all messages. This guy in the end downgraded to the previous version to "fix" the problem.First, annoying. Starting a new message, the Send button is grayed out *IF* there is anything in the Bcc field. As I normally Bcc myself on all messages, this hits every time. Clearing the field, re-enables the Send button. Refilling the Bcc field does NOT cause the Send button to go gray.
Second, more annoying, when trying to reply to a message, the Send box is grayed out and cannot be restored, even by clearing the Bcc box. I have not found any way to actually send a reply (other than to go to my mobile phone and do it there).
Third, less annoying, access to my local on-disk mailspool (referred to in Thunderbird as "movemail") has disappeared. Not a big deal, as I can review local logfiles with a command-line mailtool like good ole 'mailx.'
Fourth, and a major problem, my Address Book has disappeared. The data appears to still be there, as I can start typing an address and have it auto-completed, but the address book itself appears empty, and can't be opened for browsing.
The second is https://ubuntuforums.org/showthread.php?t=2461206 from April, so not even related to version 91 specifically. He actually got farther with the problem:
Same sort of problem. Not seeing the same thing with the "to" e-mail address showing in red, but clearly the same issue otherwise. He continues:Opened Thunderbird for the first time and all my accounts and existing emails (POP) were there. I can download new emails for all accounts, no errors.
However, My address book had disappeared - nothing in it even though the abook.mab was copied across.
I can also not send emails from any account. It gives no error, just does nothing when I click the Send button.
When I type in a valid email address in the To field and press enter - nothing happens. Doesn't recognise it (obviously no lookup to an address book) but just stays red and have to manually click in the next field - won't tab away from the To field.
Weird and frustrating.
I have just copied the .thunderbird folder to a USB again, deleted the old folder. Using Synaptic Package Manager I completely removed Thunderbird, then reinstalled. Again did CHOWN on my home folder. Still no address book in Thunderbird and cannot send emails.
Also tried replying to a received email. The address is something like First Last <firstlast@gmail.com> This appears in red and the Send button is greyed out. I have to remove everything but the contents between < and > to get the Send button to be enabled - but then nothing happens when I press it.
As I say receiving mail is no problem. I have checked the SMTP server settings and all seems fine - same settings as on my phone which works no problem.
I had Dad try this out, moving the abook.* and history.* files to a "backup" folder. The outcome of that is that half-typed familiar e-mail addressed no longer autocomplete (expected that to be the case) but unlike with this other guy, it didn't fix his inability to send. (The other guy eventually recovered his address book too, but I'll worry about that once sending is functional again.)It appears that the lost address book and the inability to send mail is linked.
I gather that Thunderbird no longer uses the .mab files for the address book - rather it now uses .sqlite files (abook.slite, history.sqlite). The presence of the .mab files is a hangover from upgrades of TB not tidying up after itself.
I closed TB, created a SAVED folder in .thunderbird and moved all .mab and equivalent .sqlite files there. Reopened TB and it had created new .sqlite file - but no .mab files. I can now send mail again, and the empty address book is getting populated from the mail I just sent.
So, at the moment I have lost my address book. I tried moving the new .sqlite files and replacing with the old .slite files but again no address book so reverted to the option of being able to send mail again.
Any clues on this? As an absolute last resort I can have Dad set up a new profile from scratch and copy all the mail files over to it, but I'm hoping for an easier fix than that. It does seem like something in the address book or otherwise got corrupted during the version update and broke stuff. (I said this happened on two computers, but I'm sure I set Thunderbird up on his laptop by copying the profile from the desktop computer. At least that means it's some consistent problem caused by the profile itself and not some weird glitch that strikes randomly.)