Rated 5 out of 5 stars

Thanks for the github tip! Here a howto:
Go to https://github.com/janodvarko/PixelPerfect
Click "download zip". Decompress somewhere. Browse to srcExtension/ folder. Search for your pixelperfect addon destination, it's about here:
C:\Users\USER\AppData\Roaming\Mozilla\Firefox\Profiles\SOMECODE.default\extensions\pixelperfectplugin@openhouseconcepts.com
Content should looke similar (chrome folder etc). Close Firefox. Replace old addon files with the ones from the zip. Restart Firefox. Smile (important).

This review is for a previous version of the add-on (1.8.0.1-signed).