I see the same issue!
I just completely re-installed Open OS, and it didn’t crash the first time it was started.
But after leaving the phone on for a few hours, it now crashes every time I try to start it…
Fennec F-Droid is F-Droid’s version of Firefox. They just throw some of Mozilla’s more dubious additions out as they build it, so it’s not called Firefox by them.
(However, it is called Firefox in the App itself, so if you are in doubt, it really still is Firefox.)
If you wanted to use Firefox and get it updated conveniently, Fennec F-Droid is your way to go.