On the prompt save xpi download the file and save to a Folder on your hard disk

Then go to Firefox
Go Tools/Add-ons
Open Add-ons Page
Restore down addons page (Make small)

Minimise Firefox

Go to Folder with downloaded xpi file
Restore down page (make smaller)

You will then have hopefully two smaller frames on your screen, the Add-ons page and the Folder with the xpi file in it

This will enable you to easily Drag and drop xpi onto the Add-ons Page

Follow prompts

Restart Firefox

Good luck !