This is not exactly a new change to Firefox 3's nightly builds since the bugs have been setting in my browser tab bar for about a week but I figured I'd write about them anyway since it's quite a nice improvement.
What's new? Download Resume - now you can pause and resume your downloads in the Firefox Download Manager. Implemented by a Summer of Code student, this brings Firefox's Download Manager up to speed with Safari (which has a really nice download resume feature that leaves
.download files that you can just double-click to resume the download).