Hi,
This solution may be absolutely useless to you but it works for me in a weird way. I sometimes have difficulties with Firefox where it fails to load a page or does so very very slowly when I start to begin browsing. However, if I load the same page in IE it loads perfectly and quickly.
Therefore my solution was to first start browsing a page with IE to get 'the engine running' then open Firefox and start browsing from there.
I've actually sat with my PC using firefox wondering why my connection does not seem to be working then open IE to find that the connection is working perfectly.
It all sounds as if Firefox was working offline but to the best of my ability it was definately active and connected to the internet but in a very slow way.
So the key, open IE, get a web page, close IE, start Firefox and it works.
Sounds like absolute nonsense but works for me.