You can also open .xpi files this way:

1: Right click on the .xpi file.
2: Select Open (located at the top of the Right click drop down menu.
3: Select the Program from a list, via the box that comes up named as Windows.
4: Select Firefox (Leave the checkbox unchecked, unless you want assign the file format with Firefox.
5: Firefox will display it's install box, once it displays "Install Now" left click "Install Now".
6: Once installed, look in Tools, and you will see Firenes just below Oprions.
7: Click on Firenes, and your done.