@horst724 I did not check, as I've assumed that they should be able to run add-ons. Else it would be a very stunted fork.
Even Pale Moon, which AFAIR is a pre-Quantum Gecko fork uses the legacy add-on schema.
Others, like Librefox, boast uBlock built-in or other kinds of privacy protections.

However I didn't do any in-depth research into this, so YMMV.