This Mozilla add-on, called Tab Overflow Scrollbar, assists in navigation by creating scrollbars for easier accessibility.
What's more, the extension also gives a vertical scrollbar to the alltabs dropdown menu, which can be incredibly helpful when browsing through extensive lists of tabs. The scrollbar only appears when required, so it doesn't clutter up your screen unnecessarily.
However, it's important to note that Tab Overflow Scrollbar is not designed for Tab Mix Plus users. If you're using that extension, you should instead select the multi tab bar options on that extension instead.
Overall, I would highly recommend Tab Overflow Scrollbar to anyone who frequently juggles lots of tabs on Firefox. It's a small, unobtrusive extension, but one that can make a big difference to your browsing experience. Plus, given that it's free and only requires Firefox as its only requirement, it's a no-brainer addition to any Firefox setup.
Version 1.8: N/A