There is the Tampermonkey addon for Firefox Android which lets you install user scripts. I found this user script which is supposed to do what you want but it doesn’t seem to work for me. Maybe it needs to be updated.

But I think in general you’re better off using a third-party app as suggested in the other comments. I would recommend RedReader since it’s open source and has been given unrestricted API access by Reddit due to its accessability features (at least for now, who knows what Reddit decides to do next).