Another point of view Rated 5 out of 5 stars
For everyone who loves this extension and still wants to use it but leave the last updated firefox as is I have a solution that works for me.
1. search on google for a portable firefox version, prefferably old than actual, for example version 13.0.0 you can try with some newer if you want. This doesn't require install or downgrade your current version of Firefox, just run it and when you are done exit. Is the portable version, you can even take it on your usb stick.
2. install pixel perfect and other extensions that you want
3... done :) happy coding
Yes, I know there can be some changes from an old version to the last version of Firefox... but in the visual part firefox is the same, not big changes, I am right?