As with all apps, they store their data below the
/data/data directory, in a directory named according to their package name -- which is what you can see following the "id=" in the playstore URL. So for Firefox, data should be in
So far the good news. Now for the bad news: This directory can only be accessed by the app itself -- or by the root user. So if your device is not rooted: bad luck.
I could imagine that for userscripts and some other profile data, Firefox might (or might not) use some storage on the sdcard -- so you could check if you see a similar directory structure there. Certainly not with the "doubled"
/data, and maybe even just named "firefox" -- as I do not have Firefox installed, I cannot check, but it's worth a look.