- Foxmarks is a free add-on for your Firefox browser that syncs your bookmarks across multiple computers. Basically, this means that my bookmarks can be accessed from anywhere, and they're backed up every time I shut down my computer.
- AideRSS Google Reader Extension: I use Google Reader to keep track of all my RSS feeds. The AideRSS Google Reader Firefox extension helps me find the best content within my feeds instantly via a simple scoring system.
- Diigo Web Highlighter and Sticky Notes: I use this one all the time to leave sticky notes to myself on Web pages, whether that be an idea I want to follow up on, research, or just commentary.
- Session Manager has saved my bacon quite a few times. It saves and restores the state of all open windows - either when you choose to restore them, or automatically at start up and after crashes (which unfortunately, happens to me quite a bit).
- Download Statusbar: This extension is quite simple; it displays your downloads in a tidy status bar at the bottom of your browser, rather than a large pop-up window right in the middle of what you're doing.
See how useful these are? What are your favorite Firefox extensions?