After fiddling around with it since you posted the link, I have to say that it looks like a very useful extension. It doesn’t use the search engines installed in Firefox, but you can add any website with a search field to its context menu, which IMO is much more convenient. The display of the search results is customizable to be shown in a pop-up window or same/new tab/window. There’s also the already mentioned conversion feature, which handles currency and several physical units, and translations between a set of over 30 languages, which can be done in the page itself or to a pop-up window.
I’m glad you found an extension to do what you want. But did you know you can highlight a word or group of words and drag it up to the search bar, and instantly get a search?
Check out SearchWith as well- you can define what site you want to power each category of search. I’ve been using it for a year or two now, and it’s awesome.
After looking at Hypersearch, SearchWith is exactly what you described in your OP.
If one is running a Gnome desktop, Googlizer is a handy little app. You can highlight text in any application, not just a browser but anything… a spreadsheet, a pdf, etc. Anything that can be highlighted works. Once the word or phrase is highlighted then just click on the Googlizer icon. In Firefox, it will open a new tab with the google results for that search.