The latest Firefox requires newer GTK than what I've got on the computers I have access to these days. And the latest Opera is ahead of old Firefox, for sure. So I'm experimenting with Opera these days.
Quick review: I want incremental search. Putting downloads ("transfers" in Opera) in a tab out of the way rather than an annoying downloads window is great. I've used Download Statusbar on Firefox, and that works too, but it's nice to have a non-annoying way built in.
And an update to my original post: Just be careful about random incompatibilities. Some sites I need (or want) to use Opera, others old Firefox. Sort of annoying.