If you run Windows in safe mode, to bypass startup apps like Malwarebytes, does TB still crash on startup?

I tried starting in Windows Safe mode and in Thunderbird "compatibility" mode (shift + start). nothing worked. I ended up downgrading back to version 102.15.1's familiar old face. All is good...for now. Thank you

The crash is an out of memory error. I think this bug report covers it, although there dop appear . https://bugzilla.mozilla.org/show_bug.cgi?id=1764865

I was going to suggest you run an integrity check on your hard disk. While the discussion in the bug is about odd memory management. These OOM things can often as likely be the result of a hard disk issues with files being recorded much larger than they are. A simple chkdsk check will eliminate that as a possibility.

The real question is what part of your user profile did you corrupt in downgrading. I have no idea, but most likely something. There were significant profile data format changes in the 115 release. Everything might be good. Now. What comes when the next update occurs is anyone's guess. There is a reason downgrading is simply not supported. Data loss is a quite likely outcome.