I held off upgrading my copy of Firefox for a couple of months, hoping that my favorite plugins would get upgraded. Today I tried to install the new blog editing tool Performancing and it failed because it was only available for 1.5. So I finally upgraded and: wow! This is only after a few minutes of use, and no crashes yet, but the speed pickup is noticeable. Layout seems to be much faster, Bloglines seems to be much faster (Bloglines is almost all JavaScript for display).

So, if you've held off upgrading, upgrade already.