I prefer to have a close button on the edge of each tab, and when I close a tab for focus to go to the previously selected tab, so these two are invaluable:
(I wonder if these actually have any code in them, or they just change some preferences somewhere)
Next, here are two extensions which help me to solve two web annoyances - pages with crazy annoying flash, and sites that are designed only for IE and don’t work right in firefox.
And depending on how much web development I happen to be doing, this one comes in handy:
Both flashblock and web developer have useful buttons you can put on your toolbar. The button for flashblock lets you add the current site to your flash-allowed site list, and the button for web developer allows you to quickly show/hide the developer toolbar.
To get the buttons, you go to View->Toolbars->Customize and just drag them onto the navigation bar.
Finally, here’s another nice one to have:
This one automates lookup to bugmenot.com for anonymous username/passwords for registration required sites like nytimes & friends, and autofills form fields.