Firefox 3’s low memory usage, password management changes, and new standard features get the majority of the buzz. Here’s a few that help in the day-to-day but don’t make it to the frontpage.
Alt text is now able to go multiple lines. Firefox 2 would cut off alt text at a certain length and dictated the need of an extension like Long Titles. Firefox 3 does this without the extension.
Source: http://www.xkcd.org
Multi-line URLs now copy+paste nicely. If a URL is in plain text and spans multiple lines, copy+pasting used to only grab the bottom most line. With Firefox 3 pasting into the URL bar now stitches the whole URL together instead of just the lowest line.
Site preferences are now remembered across sessions. If you zoom in/out on the text/images of a site with Ctrl++/Ctrl+- you can close and revisit the site and your preferences will be remembered.