but I've just uninstalled Firefox 3 (portableApps) and reinstalled Firefox 2
(I'm also a gmail user - but that's probably unrelated). FF3 was way too slow, there must have been a process hanging because the delays and page-loading were unbearable. I even bought a new USB stick thinking that perhaps the old one was dieing.
I've not had time to properly diagnose it but now that I'm back with Firefox 2 (also from portableApps) and I'm happy