Yes, yes, yes! You were 99% right, djkilla! I've decided to work on this problem from A to Z and I've tested couple of 64-bit Firefox builds. Every one of them crashed or froze. Then I've decided to assume that both XFire and Waterfox work correctly and there must be something else interfering with them. I've started to uninstall typical applications which might do that. I got my answer on the first try! Uninstalling Comodo Firewall (64-bit) did the trick! So for anyone having problem with XFire and Waterfox - check if you're using Comodo software.
Case solved! Waterfox is back on board and so far - it will be for a looooong time! Thanks everybody!
Can't believe I forgot about checking the firewall and sandboxing. Anyway, good to hear that everything is playing nicely now. Enjoy!