Ok, this is what I've done and so far is working:
After you try to re-install Firefox and the error message appeared, go to task manager (ctrl+alt+del). Under the tab 'Processes', the last (listed by name) process is the xpi... Select it with left-click and click on "end process". That should let you install FF again.
Now, in order to run it, YOU MUST right-click it every single time, and select 'run as admin..'. That gets rid of the problem.
If by mistake instead of right-clicking, blah blah blah, you just double-click the icon, don't panic. Just go to task manager again, end the xpi process and try it again (this time following the instruction).
I hope it works for you just as fine as it works for me.