Add-on asked to install Firefox

I get always the message when I will install an extention: Please download firefox and install

This is on Do you have something changing your useragent?

Yes, this is from the mozilla addons site. I don’t think I have change something.

Some security software and some add-ons may change your user agent string. You can see what is being sent to a website by checking the Request Headers in the Network Monitor. If it doesn’t fit the pattern the Add-ons site expects, you get the wrong button.