Macromedia Flash on Web sites while using Firefox? Try installing Flashblock. You can install the .xpi file as a user and have it work at sites like Tom's Hardware as soon as you restart Firefox.
Another cool plugin for Firebird (or Mozilla for that matter) is Live HTTP Headers. You can use this plugin to watch your browser's HTTP requests and the server's HTTP responses. To install this plugin, change the permissions on your /usr/X11R6/lib/firefox/lib/mozilla-1.6/components and /usr/X11R6/lib/firefox/lib/mozilla-1.6/chrome directories to be writable by the user installing the plugin. Alternatively, run Firefox as root and then install the plugin.
Update: Check out the Web Developer Firefox extension. It's incredibly powerful.