Rated 4 out of 5 stars
I've had this extension installed for months, and while it works as advertised and it's pretty cool, truth is that my Google searches rarely have a Wolfram-Alpha equivalent, so I've only ever seen the W-A results on the right side of the screen a handful of times. But that's not the extension's fault. So I say good job to the author for the idea and to readers of this review who haven't yet installed the extension, it's worth a look, at least, so give it a download and give it a try.
One minor complaint: Please don't force users to add an icon to their statusbar - provide an option for statusbar icon OR toolbar icon. Thanks.
Option to hide Statusbar icon
Thanks for the feedback. It seems Wolfram Alpha is gradually adding more content but yeah it doesn't have results for a lot of categories and could do with a lot more.
About the statusbar icon. There's a new version (1.0) available now that adds an option to hide the statusbar icon. You can get it from this page now https://addons.mozilla.org/en-US/firefox/addons/versions/12006 or wait until it is approved by Mozilla and it will update automatically.