While the majority of Firefox add-ons install automatically when you go to install the add-on, some don’t always. Further, Thunderbird doesn’t even have the functionality to do an automatic add-on install. So in those cases you have to download the add-on, find it on your computer, open Firefox’s or Thunderbird’s Add-Ons Manager window and then drag the file into the window to install. There is an easier way, both Firefox and Thunderbird have an ‘Install’ Button on their add-on managers, but by default it is turned off. To turn the button on requires a very simple tweak in the program’s…