Rated 5 out of 5 stars

Workaround:
1) right-click on the greyed out button "Add to Firefox" to download the XPI file.
2) open XPI using archiver software (i.e. WinRAR)
3) edit install.rdf (with Notepad) and change maxversion to 4.*
4) save change and update archive
5) install extension (in FF) via File > Open File (or CTRL+O)
6) restart browser

This review is for a previous version of the add-on (0.4.7).  This user has 2 previous reviews of this add-on.