First of all, Hindi rendering actually works. What annoyed me all the time in Firefox 2 was that Hindi rendering worked on some sites (such as the Hindi Wikipedia) but not on any blogs that I visited. This leads me to believe that the Hindi Wikipedia is encoding the Hindi in some friendlier way that is less prone to looking like a pile of trash. In any case, after installing Firefox 3 B2 on Windows, pages like this, this and this now look just fine. Phew, big relief!
The second most noticeable thing is that the address bar is a lot more active now. If you start typing something in it, it not only finds websites that have the same domain name as what you're typing, but also searches the entire URLs and the web page titles for all the web sites you've visited and shows them right there. Neat! Also, all windows now have an address bar; mainly, this means that pop-up windows show their addresses (for security reasons, presumably), although the field is not modifiable, so you can't type in an address.
There is now an 'Applications' pane in the Firefox Options which lets you decide what happens when you encounter files of a certain type on the Internet (whether they are handled by a plugin, opened with a certain application or just downloaded).
There are also some changes to Bookmarks. In order to bookmark a page, you can still use the age-old Control+D shortcut, or you can click the star icon that shows up in the extreme right hand side of the address bar; the star is empty if the web page is not in your bookmarks and filled with yellow if it is. In either case, a little pop-up box shows up in which you type the name of the bookmark, where it's stored, and here's another new feature, any tags you might wish to associate with it. There is also, by default, a folder called Smart Bookmarks in the toolbar which has "Most Visited", "Recently Bookmarked" and "Recent Tags", all of which show what you'd expect them to. If you choose "Show All Bookmarks" from the Bookmarks menu, you'll see the new Places Organizer which is basically the new Bookmarks Manager, and one cool addition is that it has a Search Box.
The Downloads window also looks nicer and has a Search feature. In general, one of the themes in Firefox 3 seems be adding Search to as many places as possible (Places Organizer, Applications pane in Options, Downloads) and improving it where it already exists, such as the Address Bar.
(The only bug I've encountered so far has been while writing this entry, which is that when I inserted an image into the Blogger post editor and cut it to the clipboard, my cursor disappeared and has not yet returned!)
[UPDATE: I tested Firefox 3 Beta 2 on a Mac also, and I'm extremely happy to see that it also benefits from both the Devnagari support and the dramatically boosted JS performance.]