It sort of works, sometimes. Often times the video will start off at the preset speed, but about 30 seconds in, drop back to standard 1.0 speed. If you use the same tab to start another video (click on related etc.) it never works and will always play at the last speed youtube thinks it played at (1.0 unless you've manually adjusted it), so more often than not, this doesn't work. It's nice when it randomly chooses to though. Hopefully the developer fixes the bugs.

While it allows new defaults, YT natively supports .05 increment speeds as "custom" speeds. I'm still looking for exactly fractional speeds.

The Popup interface could be improved a bit (increase by 0.1 instead of 0.05 or change it to a slider that goes from 0 to 5), but the extension deserves 5 stars for:
- being open source
- the code is stupid simple and a good example for anyone who wants to look into writing Firefox extensions or js in general
- as a result of the first two points, packaging the extension with changed properties (if you want to) is actually pretty easy.