I've become addicted to, or even dependent on, FF's extensions and themes. I don't think I can truly switch to any other browser now... period. Unless they port the same functionality that I depend on through extensions. I use Opera for quick email checking or something because of speed dial.
But anyway, I don't think I can live w/o Foxmarks extension. I switch between my laptop in dorm and PC at home and exporting/importing bookmarks back and forth to keep them the same is such an annoying hassle... I used to do it. Now Foxmarks is heaven by comparison. That's #1 necessity but also Taboo, ScrapBook, InFormEnter, and Flash Killer are some other really important ones. The list goes on.