located it under:

/data_mirror/data_ce/null/0/org.mozilla.firefox/files/mozilla/5476rl0j.default/extensions

I’m on rooted PixelOS and only thing I have under data that’s related to firefox are:

Android -> data -> org.mozilla.firefox -> files -> Downloads

and there is 0 files

Total Commander didn’t show any extensoion file in none of:

Android -> data [ -> org.mozilla.firefox ]
        -> media
        -> obb

these (and I have several: uBlock, clearURLs, Canvas, …)