The new FF extension is done and will now need to be reviewed and approved by Mozilla. I’m aware that the extension is currently not operating (visible) on Firefox 40+ so please use the generic bookmarklet available on pinsapp.com in the mean time! It will do the job just fine until the official add-on is available.
[UPDATE] The Firefox Addon has been approved. FF users rejoice! Pins will now work smoothly from within Firefox again.