With a bunch of tabs open in Firefox, you can cycle through the tabs with Command-1, Command-2, Command-3, etc. This will select the respective tab, starting with the leftmost one. This is great for laptop surfing. I just happened upon this accidently when reaching for Command-W to close one, and hit Command-2 instead. Great!
[robg adds: You can also use Control-Page Up and Control-Page Down to cycle through tabs in order, in case you have more than nine open at once. I'm also running this hint to point people towards the Firefox built-in help system: Help -> Help Contents from within Firefox. If you haven't visited it lately, it's come a long way since the pre-1.0 releases. The above keyboard shortcuts are documented in the Tabbed Browsing -> Using Tabbed Browsing section. There are a lot of useful tips in the help, and it's well worth the reading time!]
Mac OS X Hints
http://hints.macworld.com/article.php?story=20050114201044946