It gives you a button to defragment your Places database, which might greatly speed up your awesomebar. You can also set it to run automatically at startup after the browser has been restarted X times (by default X is 50, and I'm not sure that I restart Firefox 50 times in a year, so you may want to reduce this).
https://addons.mozilla.org/en-US/firefox/addon/vacuum-places...
It gives you a button to defragment your Places database, which might greatly speed up your awesomebar. You can also set it to run automatically at startup after the browser has been restarted X times (by default X is 50, and I'm not sure that I restart Firefox 50 times in a year, so you may want to reduce this).