I personally prefer Librewolf or Abrowser over IceCat, but if you can get IceCat to work then great.
You should be able to get extensions from the Mozilla Add-Ons Store. Failing that, the GNU Project has Mozzarella.
On Android, I would only recommend IronFox. It's available on GitLab, Accrescent, Obtainium, and F-Droid (third-party repo).