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Stylish will Now Install. Rated 5 out of 5 stars
O.K. Found out what the problem is and everybody take notice.
This is Not a Firefox error!
The reason why some Add-Ons, including Stylish will not install as in my case, is because my KIS Firewall was blocking the installation.
Disable your AV, including the Firewall and everything will go as it should.
Darn, so simple!!