If you decide to upgrade from Firefox 2 to the first release candidate of Firefox 3, you'll notice an improved performance, less memory leaks, a revamped interface and some very interesting new features:
* select discontinuous text - press Ctrl while selecting different blocks of text, like you can do in Microsoft Office
* I've always liked about Notepad2 that you can select a text and open the Find dialog to search for your selection. Firefox 3 brings this feature to improve its already powerful find-in-page.* in Google Toolbar 4, Google made bookmarking a very easy task: just press on the star button to bookmark a web page and press again if you want to add tags or edit the title. The latest version of Google's plugin complicated the feature a little bit, but Firefox 3 borrowed the initial concept and changed the way you bookmark web pages. Furthermore, you can now associate tags to bookmarks and find the most visited bookmarks.
* the address bar auto-completes URLs and page titles, while trying to prioritize web pages visited frequently and recently. "Dubbed the AwesomeBar, it lets you use the URL field of your browser to do a keyword search of your history and bookmarks. No longer do you have to know the domain of the page you're looking for — the AwesomeBar will match what you’re typing (even multiple words!) against the URLs, page titles, and tags in your bookmarks and history, returning results sorted by frecency (an algorithm combining frequency + recency). Not only that, but the drop-list results show you the page's favicon, the full title, the URL, and whether you have bookmarked and/or tagged the page in a richly formatted two-line display."
* disable plugins from the Add-ons dialog. That means you can now disable Flash or the Windows Media Player plugin directly from Firefox, without having to install extensions like FlashBlock.* if the current web page uses OpenSearch to point to a search engine, Firefox shows a blue circle next to the search box. You can add the search engine to your list using only two clicks.
* you can add web applications as handlers for protocols like mailto. That means you can open all the mail links in Gmail without installing an extension if you follow these instructions. Hopefully, in the next version of Firefox you'll be able to also open other kinds of links in web applications (for example, open PDF files in PDFMeNot).
* set permissions for individual web sites: for example, you can disable images or cookies in some web sites you don't trust. Opera offers more fine-grained options in "edit site preferences", but you may still find useful the limited permissions from Firefox (Tools > Page Info > Permissions tab).
* another feature already available in Opera, full-page zoom, has been included in Firefox 3 to allow the scaling of images and other objects. To revert to the zooming feature from Firefox 2, enable View > Zoom > Zoom text only.
Even if Mozilla didn't release the final version, Firefox 3 RC1 is an almost-final version and could safely replace Firefox 2. There's also an unofficial portable version you can use before installing the application. While not all the extensions have been updated to include support for Firefox 3, you can easily disable the compatibility checking.
All in all, Firefox 3 added some cool features, but it didn't ignore fundamental issues like performance or a better back-end for web history and bookmarks. The new interface and the sometimes-not-so-awesome bar are two of the things that still have rough edges.