That's even weirder !! The mere opening of Firefox does something that
makes the router lock up but if FF is already open and the router is
re-booted then access comes back. Reading back you've already tried
the obvious which is changing the router but for completeness try
changing the network cable between the Ubuntu system and router. A
very long shot but you've got into that sort of territory now
What you need to look at is what's common to everything you've tried
so far - even using a different network card ( or fitting a card if
using on-board, or a different wireless card if connecting that way ).