It's probably related to this: https://gitlab.com/relan/fennecbuild/-/merge_requests/63
F-droid Fennec had build problems lately due to google removing big dependencies from its android package repo or whatever, so it's well out of date for now. The latest version there has at least that one well-known security problem that was in the news a few weeks ago. I don't know why you're getting notified about it now, I have it installed and didn't see that. But if you're risk-averse then you probably shouldn't currently be using it to visit websites that might be malicious.
Recent comments over there suggest that progress is being made at last.