TabNavigator Brings Alt+Tab Style Navigation to Firefox:

Firefox: Looking for a visual way to quickly navigate between tabs in Firefox? TabNavigator brings Windows-style Alt+Tab switching to Firefox.

After the default installation TabNavigator enables Alt+R and Alt+W for forward and backward navigation through your tabs. You can modify the navigation keys to be any combination of Ctrl, Alt, or Shift + another key.

The initial preview is simply the favicons of the sites plus the title bar text at the bottom of a small gray box. You can increase the size of the icons or switch from icon view to full preview. Instead of the favicons you'll see a preview of the actual page. The default preview size is 500px, which isn't a bad size for previewing—although if you frequently tab between pages that are very similar looking, you may want to increase the preview size for clarity.

TabNavigator is free and works wherever Firefox does.