I get always the message when I will install an extention: Please download firefox and install
This is on addons.mozilla.org? 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.