Firefox (29+): The new Firefox 29 has redesigned the tab bar, and tabs now have a fixed width. The Small Tabs extension makes them flexible so you can fit more tabs in the title bar.
In Firefox 29, if you open more than the amount of tabs that fit in your bar, the browser puts arrows on either sides to scroll through those tabs. It's not efficient use of screen space and makes it slower to use.
Small Tabs reduces that fixed width of tabs so that more than the usual amount can fit. On a 1366-pixel wide screen, Firefox 29 could fit 13 tabs in default mode. With Small Tabs, that number increased to 34 -- a significant bump.
It's neat, it's light, it's free. If you're using Firefox 29, it's definitely worth a try.