So, I upgraded to Firefox 1.5 not knowing that you couldn't easily install it side-by-side with 1.0. Most things work ok, though there are some annoyances - but there's one HUGE annoyance, which is how they changed the way page errors and warnings are handled. It used to show the JS errors in the Console window, which made it pretty easy to skim. With 1.5 they decided to include CSS errors as well, making the list extremely long and annoying.
Well, today Greg at work found a great extension to fix it, called Console 2 (squared). It allows you to filter the results into JS/CSS/XML, choosing any combination of the three. I honestly don't know what I'd do without it. Now if only they would make the classes in the DOM Inspector copyable so I didn't have to keep retyping them when I'm debugging a page...
Console² on Mozillazine Forums