I’ve updated my Firefox to the latest version, but when I want to install this add-on, it shows me this error.
What’s your Firefox version ? The addon should be compatible with Firefox versiosn above 60.0.
This message probably is related to your browser’s user agent string. A typical Firefox 96 user agent string looks like this:
Mozilla/5.0 (Windows NT 10.0; Win64; x64; rv:96.0) Gecko/20100101 Firefox/96.0
The Add-ons site is very sensitive to that last bit and may show various strange responses if it doesn’t follow the expected format.
To see what user agent your Firefox sent to the site, open the Network Monitor in the lower part of the tab. (Ctrl+Shift+E, or on Mac, Command+Option+E; MDN)
Then reload the page. Click any request and on the right side, in the Headers pane, scroll to the bottom to find the User-Agent under Request Headers. Anything strange there?
Note: one program recently found to be changing the end of the user agent string is Avast/AVG AntiTrack.