Go Directly to Your Favorite Sites:

You can add keywords to your bookmarks for easier and faster access. From the Library, just add a short keyword in the keyword field, and you’ll be able to access that bookmark by simply typing that keyword into the address bar. For example, you could give your account the “links” keyword, and from then on simply typing “links” into the address bar will take you right there.

Create Smart Folders:

If you’re an advanced Web surfer who frequently needs to save and track a variety of specific sites, here’s a useful time-saver: using Firefox 3.5’s Library, you can create and save searches into folders that are then automatically updated as you add sites to your bookmarks and history.

First, open the Library by selecting the "Organize Bookmarks" option from the Bookmarks menu. Then, enter your search terms in the search box. Then click the Save button to create a Smart Folder.

Manage Your Downloads:

If you’re a frequent downloader, you can use the download manager window to keep track of all your downloads. You can pause and resume downloads, and save yourself time by opening files directly from the manager.

If you need to track down a past download, go to Tools → Downloads and use the search box to find your file. Once you’ve found it, double-click on the file to open it, or ctrl-click and choose "Copy Download Link".

Discover Developer Tools:

If you’re a Web developer, Firefox’s developer tools will make your life easier.
The Mozilla Add-ons site offers many tools to streamline the development process, including Firebug to edit, debug, and monitor CSS, HTML, and JavaScript live in any Web page, Tamper Data to view and modify HTTP/HTTPS headers and POST parameters, and the DOM Inspector to examine any HTML or CSS element with a simple right click.

Set the Web Up Just Like You Like It:

Now you can use Web-based protocol handlers to provide quick access to your favorite Web applications. For example, you can set Firefox up so clicking on a mailto: link on any site will open a new message in your preferred webmail provider rather than your computer’s default mail program (note: this feature is only available with webmail services that have registered with Firefox 3.5).

Go to Firefox→Preferences→Applications to select the default application for each protocol or to select “Always Ask” if you prefer to choose the application yourself every time.

If you’re a Web app developer, check out more about how to add support for Web-based protocol handlers in the Mozilla Developer Center.