Brilliant, but the icon's a tad borked. Rated 4 out of 5 stars
A brilliant add-on, for those of us too impatient to wait for Mozilla to implement something like this in Firefox proper.
One gripe I do have though (and it would be a bigger gripe if I hadn't formulated a temporary fix myself*) is that the toolbar icon is displayed wrong in both the default theme packages with Firefox in Ubuntu AND my preferred theme Jupiter (by zeeeeee). Instead of displaying the themes default download icon, the toolbar button displays a partial of the new tab and new window icons.
*As I alluded to above, I've formulated a quick fix via a simple userstyle (see below). It works perfectly for large icons, but not for small icons. Alas, I'm sufficiently out of practice that I haven't figured out (yet) how to get it to work on all toolbar scales. The userstyle fix is as follows:
-moz-image-region: rect(0px, 24px, 24px, 0px) !important;
I'll probably upload this to userstyles.org once I've figured out how to get it to work on all scales. It'd be nice if this seeming incompatibility could be investigated and rectified within the add-on itself. Hint, hint. ;-.