keep losing customized CSS

Not sure why or what event triggered it, but twice in the last month I've lost the customized (minimized!) CSS I use. Last time it happened, I was able to find where I had stored it on another computer, but I cannot find it this time.

First, in which file would I find it stored in my thunderbird profile directory? Second ... not sure what would cause it, but ... this is a bug, this customized attribute should persist sanely.

I'm using 128.6.1esr, and MDHR-4.0.3.3 (dec 29, 2024).

Thanks!