https://blog.mozilla.org/blog/2019/05/09/what-we-do-when-thi...
About alternatives: as Ultramanoid said, there is no other decent browser. If you want privacy and respect your only choice is Firefox. Everything else is Chromium based (or as i would put it: "spyware based").
(Edit: You naturally have a few browsers based on firefox as well, such as Tor Browser, Waterfox and so on... but Firefox is the real deal ;)
I use Firefox Focus most of the time instead of the full blown Firefox, and its so stripped down that it doesn't support addons (and so wasn't affected by the cert issue). In fact it doesn't even really support tabs. It does help me not have a trillion tabs open and focus on one thing at a time. Depending on your browser needs it's a good, albeit totally different, experience.
Still, latest version is fixed.
If you want you may try the VERY experimental version of Firefox Preview ( https://www.apkmirror.com/apk/mozilla/firefox-fenix )
You can try to fix it from about:support, but beware that it might delete some preferences, logins and other things like that...
by the way, you also have GNU IceCat Mobile -> Firefox minus proprietary things plus security measures ( https://www.gnu.org/software/gnuzilla/ )
That said, try Bromite : https://bromite.org
Edit : Unclear from your question if you're aware that the problem was fixed -- just update to the latest version.