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.
:-(

In my PC, Thunderbird sometimes starts with a new profile. I can restore all the profile and emails from the .thunderbird folder as it creates a new profile but keeps the "old" profile. So I move all the data to the new profile and everything starts working fine.
I don't know if your problem is similar to mine, but it seems a bit similar. Maybe the symlink created more issues in your system, but I can confirm Thunderbird sometimes has strange behaviors. I have no idea if it is a Thunderbird or KaOS issue.
Not a big issue for me, but in your case it seems pretty annoying.

For both of you, KaOS does nothing for Thunderbird, it uses an upstream provided bin (so no extra GTK libs are added to the repo). It is not build here (from source), just unpacked the bin so you can access it.