ff3sudoku is an add-on that features an engine for playing Sudoku puzzles. It can generate random puzzles, but it also detects and imports Sudokus from several Web sites, including websudoku.com, sudokupuzz.com, sudoku.com.au, and others. There is also a stand-alone version working in Firefox 3+ and GoogleChrome.
Showcase is a Mozilla Firefox plugin that allows tabs and windows to be managed by showing them as thumbnails in a single window, tab, or sidebar. It includes a find bar that will filter the thumbnails and the capability to select the thumbnails in the same way you would select files in your system.