Koroush has updated his Firefox tweak guide to fully detail the changes that have come along with the release of 1.5. If you've installed 1.5 & are noticing these differences, TweakGuides might just have the answer for you.

"To coincide with the release of Firefox 1.5, I've fully updated my Firefox Tweak Guide to cover the features of this latest version. If you've read the guide before, give it a quick run-through to see what's new, and if you're new to the guide, I think you'll find it very handy."