As explained in https://github.com/mozilla-mobile/fenix/issues/10618#issuecomment-697107024:
Select text range, just like any other app.
Firefox selects just one word, and then you need to use the handles to select the rest.. and it works weird because the nav bar moves.
See it in action. Left side is Samsung browser 12.1.2.5. Right side is Firefox 81.1.1 (Build #2015764547).

Can you check with Nightly? There are some touch scrolling fixes that might improve this situation. I did a quick check on Pixel 2XL running Nightly and did not see the address bar move with the text selection handles for me on https://paste.mozilla.org
I just tested. It's much better as it doesn't seem so crazy, but it is still away from normal Android experience.
Firefox selects just one word, and then you need to use the handles to select the rest..
This still happens.
and it works weird because the nav bar moves.
This is fixed.
Selecting one word is standard Android behavior - it's also what chrome, gmail, and other android apps do. Will close since the navbar dragging is fixed, and the others are Android behavior.
Nope. On any other Android app, this is what you do with your finger to select a text range:
Firefox needs:
It's obviously not standard. Just check the Samsung browser part of https://github.com/mozilla-mobile/fenix/issues/15391#issuecomment-698421694.
Chrome on Android 8:

Apps usually allow you to select more than 1 word.
Sorry about that! Let me move this to our GeckoView team to see what we can do there.
Moved to bugzilla: https://bugzilla.mozilla.org/show_bug.cgi?id=1671530
Change performed by the Move to Bugzilla add-on.
Most helpful comment
See it in action. Left side is Samsung browser 12.1.2.5. Right side is Firefox 81.1.1 (Build #2015764547).