How to install .xpi add-on file for Firefox?

Solution 1:

Go to tools>addons (or open addons from the firefox button, or type about:addons in your address bar and press enter).

• Now, click on the button:

  

• And click install addon from file:

  

• Then locate your addon in the file manager and install it :)!

Solution 2:

Drag'n'drop from Nautilus or whatever else to Firefox window should work too (any tab, doesn't need to be addon manager).

This way is somewhat faster if you already have found .xpi file.

Solution 3:

You can just open FireFox and then drag the *.xpi File to the fireFox Browser, then it will automatically open the *.xpi file.