I use Firefox because I have noticed a difference in the speed. But there's one problem with it - which may be Firefox or just my crap computer, but every time I goto a secure site i.e. online banking, it just disappears and you have to load it back up - this can continue for another 3 or 4 tries.
Have use Firefox near enough since day dot with NatWest, and yet to experence any problems like you describe.