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CHAMP0This is a Thunderbird forum, so I assume you wanted to install in Thunderbird, not Firefox.
In Thunderbird, Tools|Add-Ons→Install
Thunderbird and Firefox share some code, particularly the mechanism for add-ons.
In Firefox, you can visit an Add-On's site, click on the button and the item will be downloaded (copied to your computer) and installed into Firefox.
If it's a Thunderbird Add-On, then we don't want to install it in Firefox. So, right-click the button or link, do "save as" and download it. Then you can use the Install button in TB's Add-Ons manager window to install it in TB.
Alternatively, if you can arrange things so you can see both your browser and TB's Add-ons window, then try dragging the download link/button onto TB's add-ons window. This will download and install in one go.
If you're uneasy about arranging the windows and worried about where downloaded files land, you can do a halfway house of dragging the button/link from your browser to your desktop, then dragging from there to TB's Add-Ons window.
We don't need to try to "open" xpi files. Simply feed them to Firefox or Thunderbird, and they will do all the work for you.