First off, sorry this is here. To register with Mozilla and ask for support there, I need my e-mail working, but my Mozilla Thunderbird e-mail client suddenly isn't working, so I can't do the e-mail registration. C'est la vie.
I'm using Linux Mint 18.3 Cinnamon 64-bit. The last updates were applied on the third of May, and did not include Thunderbird. The last Thunderbird update seems to have been off the bottom of my Linux Mint updates history (6th April). Thunderbird version is 52.7.0 (64-bit). I've obviously checked e-mails many times in that timescale, including several times today.
I've been using Thunderbird for a few years on this Linux Mint install, running through updates from Linux Mint 17.something (profile originally copied from Windows 7). The profile is in the ~/.thunderbird folder, and seems consistent with the profile.ini. I haven't touched this since transferring the profile a couple of years ago, until after the problems started about half an hour ago.
The problem - I launch Thunderbird and get the "Welcome to Thunderbird" dialog that wants to set up new e-mail accounts. It seems to be ignoring the existing profile.
However, it doesn't create any other folder or profile - except that after I updated my backup (which I realise I stupidly haven't updated for quite a while) and tried shifting the profile to another folder (apparently Thunderbird on Linux can sometimes use a ~/mozilla-thunderbird folder) starting Thunderbird recreated the ~/.thunderbird folder with a new profile.ini and profile. Obviously I deleted that and moved the original back, but still, Thunderbird is acting as if there's no existing profile (with 450MB of important e-mails that I need) yet not creating a new profile folder when I start it up.
Is there e.g. any way to check a profile for corruption, figure out how that could have happened, and maybe repair it?
There do seem to be a files in the ~/.thunderbird/Crash Reports folder, but the filenames all start with "InstallTime" (I also don't remember any crashes at all) and the most recent is "InstallTime20180326200027".