Of course they can be disabled. But sites are not all built the same way, so there isn't one silver bullet we can tell you about to disable them everywhere. On mobile it's even more difficult because access to browser plugins is more limited, along with developer features within the browser.

I also use FF on Android, and until recently was also using Reddit. There isn't a good way to remove that bar.

Firefox on Android only allows a select few add-ons unless you install a nightly version or something. I don't do it myself but it should be easy to Google

On Firefox Beta/Nightly you can enable Debugging Menu, by going to About on settings, and tapping the logo 3 times.

There you can add Custom Add-on collection.

You can create developer account on Mozilla, and add any add-on that you want.

Fuck Reddit