They're in the profile folder:
/data/data/org.mozilla.firefox/files/mozilla/xxxxxxx.default/
I think they're in browser.db which is an sqlite database, but I'm not
sure.
If your device is rooted you can copy it - or the whole profile. If it's
not rooted you can't.
You can sync it to a desktop machine and backup the bookmarks there,
which should include the mobile ones.
BTW, on you should be able to see the profile folder by using this URL in
Firefox itself:
file:///data/data/org.mozilla.firefox/files/mozilla/
Depending on the version of Android you may have to give the Firefox App
'storage' permissions in settings.
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