Any site that hijacks the back button to show you ads is similar to a creep locking you briefly in a room so you ‘consider’ their offer.
I think Firefox on iOS uses webkit and doesn’t support addons. There is firefox focus which does block ads but also only supports one tab at a time.
https://www.macrumors.com/how-to/enable-content-blockers-saf...
IIRC because I think that's what I did before I had a PiHole doing most of that, but I can't remember for sure.