Opera 8.51
Doesn't seem to be as 'holey' as Firefox (they bring out an update every few days it seems), although it could be said that because Opera doesn't update as often, it might be slightly less secure.
I would defo go with Opera, but as a very close second Firefox (the latest one whatever version it is at the moment).
Along with a reliable and named brand Firewall, and NAT router just to be sure (if software firewall has problems, NAT should be secure enough).
And, as Wanderer says - AV - just to make sure your not logging on with a key logger/trojan watching every move you make.