I previously used Random Agent Spoofer with Firefox on an iMac. This no longer works after recent updates of Firefox.

After an online search I decided to try Chameleon for Firefox. According to Firefox preferences I am using Firefox Quantum 61.0.1 but Chameleon refuses to download because it advises I am using Firefox 50.0. Other websites sometimes tell me to update my browser when I am apparently already using the latest version of Firefox.

Why is there this discrepancy and how do I obtain and install Chameleon?