As a Firefox developer, I'm not sure why we'd want to do that.

Forking isn't cheap if you want to make significant changes to that fork. We'd be better off staying with Gecko.

(Oh, and we're still all-in on Gecko)

why is a redirect after the submission of a login form needed? Usually they say, that's to avoid the misuse of the history function to login. But firefox already knows, that there is a password field and could clear the data after the submit. Why it's not done?

I don’t either, but the option is there if it’s ever needed. I guess I wasn’t as clear with my point for Firefox (which was a very different point for Microsoft).