Firefox... what happened to you? Consuming gross amounts of memory, slower and slower as releases go by. You were supposed to be a slim browser usurping Mozilla by virtue of simplicity, shedding the feature creep and lack of engineering that had convoluted the Mozilla suite. Now you have become the very thing you were created to kill: a bloated browser.
I installed Opera today. It may be closed source, but it's a breath of fresh air - fast, snappy, simple. Innovative features (speed dial, notes) and a clean, minimal UI. In general it just works and is a lovely UI experience.
Another option in Ubuntu/Gnome is Epiphany but it just doesn't have the polish that Opera has.
I originally posted these thoughts in the Ubuntu forums here.