Here is an interesting thing, yesterday I made 3 attempts to pay my Vodafone BB account by credit card, all was well filling out the details but upon hitting submit button I was told that internet connection had failed (3 times) and to try again later.
This morning I tried again but exactly the same results so I started up my old lap top which is still running XP and Opera plus AVG free anti virus which I run on both PC, I got straight through filled out details, hit submit and success.
Did you try using a different browser on the Vista PC? If not, then there is no point in blaming Firefox for your internet connection problems. (I wouldn't be using Firefox if it didn't work.) Have you tried resetting the modem that the Vista PC is connected to? Also, are you quite certain that the Vista PC has AVG Free and not a "free trial" of AVG Internet Security, which would include AVG Firewall? (I'm a former AVG Free user who will never go back.) BTW I suspected you were an XP user when you mentioned "Vista SP3" earlier. (I thought Microsoft decided to call that Windows 7 - LOL!!!)