The "vPlayBackSpeed" is an amazing tool which really boost up the productivity when watching videos. Some sides have a similar feature, but are often limited to -0.5|+0.25|+0.5|+1.0| speed --> while vPlayBackSpeed gives the freedom to increase or decrease a video with +/-0.25 steps between 0 (stop) and 5 times the speed.

It's very handy to skim through videos to get the whole picture or simply to find what you are looking for without jumping back and forth. Works with Youtube and several other sites (not everyone though).

A new favorite for FireFox!