I'm sure it's a titanic job: cut xulrunner out of 4.0, keep conversations, ovi sign-on and ovi maps compatibility, make it being polite to the watchdog (not sure if that's a problem), etc...

As far as I know, browser engine switching was easy in Maemo 4, there was a package for that, maybe you'd try to take a look at its source? But for sure that's kind of an incomplete workaround for us and I don't see any big difference from Browser Switchboard in Maemo 5.