The Tab Mix Plus extension has a gem of a feature buried deep within the settings: The ability to turn the Ctrl+Tab key from a direct tab switch into a popup menu that works similarly to the Windows Alt+Tab feature.
I use del.icio.us to store all of my infrequently used bookmarks, but I've found that browsing by tag just isn't very efficient for me. I always use a full text search through my bookmarks to find what I'm looking for, so what I'm really looking for is a single-click save solution similar to the star button on the Flock browser.
Interclue is a Firefox extension that lets you preview whatever a hyperlink on a page is pointing to. Unlike other link previewers, Interclue doesn't just display a tiny replica of what's hiding under the link. It uses algorithms to intelligently construct a summary of the target page and displays it in a window with lots of other information and statistics about the page.