First it's HTTPS everywhere, not HTTP.
Firefox prevented the site from asking you to install software
By default, Firefox permits you to install add-ons only from trusted websites. If the website from which you are installing add-on is not on the trusted websites list, a yellow bar appears across the top of the web page with a message similar to the following:
Firefox prevented this site (www.example.com) from asking you to install software on your computer.
To allow the site, click Allow in the yellow bar.
To add the site to the trusted sites list:
At the top of the Firefox window, click on the Firefox button (Tools menu in Windows XP) and then click Options
Select the Security panel.
At the right of Warn me when sites try to install add-ons, click Exceptions....
In the Allowed Sites - Add-ons Installation dialog, the list of permitted sites is displayed. The current site URL is inserted into the text field.
To allow the site, click Allow. Then, click Close.