I cannot install this add-on! Rated 3 out of 5 stars

Please explain how can one install the file mozillazine_search-20100528.xml

Handy Rated 5 out of 5 stars

To clarify what this does:
It adds a new search engine along where Google and Yahoo etc goes in the "search bar.

I think this may come in handy once in a while. Instead of distracting my attention and going to the forum, then pasting what I'm searching for, Then going to the results, I can do this on the page I'm on. My searches open in a new tab so I can put them side-side
First for review!