Not sure how I did it. Installed latest Kaos with, mainly to have a look at Plasma 6, and started setting it up. The first thing I did was to get Thunderbird working. I have a few distros on my PC, and I have the .thunderbird profile directory, with all my mail and settings, in a separate partition.All the distros have a symlink to this directory in their home directory, so they all access the same emails. Bit of a weird setup maybe, but it works for me.
So I did my usual thing. Created a directory called /TB, and added a line in fstab to mount my .thunderbird partition to /TB. I then created a symlink from /TB/.thunderbird to .thunderbird in the home directory. Fired up Thunderbird, and it offered to create a new profile for me, instead of opening the profile in the .thunderbird directory. I verified that the partition with the .thunderbird directory was mounted at /TB, and that my thunderbird profile was available there, and the symlink was correct. It was all good, but I still could not get thunderbird to open my profile. So I had a look in the profile, and found to my horror that ALL the files had disappeared. There were lots of folders still there, but no files. I checked through the folders to see what else was missing, and they were all empty. So I have lots of folders but no files. I checked an old profile that I hadn't used in years, and it was basically still intact. Oddly, thunderbird -p listed the old profile in the list for selection, but if I selected it thunderbird still offered to create a new profile, so not sure what's occurring there.
Anyway, bottom line is I have lost my entire email history, which went back a number of years. I've also lost all my settings, filters, and everything else. Yes, I back up weekly, but somehow I had not added the external partition with my emails on it to the backup list. I'll know better next time.
So I guess this is more a warning than anything else. Beware symlinking your profile in Thunderbird. And make sure you've got backups.
:-(