Why is this not part of Firefox to begin with??? Rated 5 out of 5 stars

Agree with the other reviewers - why is this not part of the Firefox browser to begin with??? I love the extension, but it absolutely will not work on Firefox 4. Fortunately, there appears to be a replacement in the works, if you are not afraid of experimental add-ons:
https://addons.mozilla.org/en-US/firefox/addon/opendownload-10902/