Opera has a nifty feature that can reorder the tabs by using drag n drop. Firefox lack of this, but soon it will be available in the next Firefox 1.1 release, which i don't know yet. Here is another feature, taken from Gemal's blog :
"Another really cool and important checkin is the landing of the groundwork for the new Software Update Service. The new Software Update Service will support binary patching and I think it's based on bsdiff and bspatch. This will allow Mozilla Firefox and Mozilla Thunderbird to be updated with security patches without having to download an entire new version. It will be possible to apply patches. Great work from Ben Goodger, Benjamin Smedberg and Darin Fisher."
I have been looking for this feature since my Internet connection is rather slow, so download the full file would take a long time.