It’s good that there’s an extension for Firefox called Add to Search Bar that will add a new context menu item to let you create your own search plugins on the fly.
For instance, you can right-click on the search box here and then choose Add to Search Bar…
You’ll be prompted to give it a name, I chose to shorten the site title down a bit.
And there we go, now I can search the How-To Geek from Firefox.
I’ve tested this on a number of sites, seems to work pretty well… at some point I’ll stop being lazy and make an official search plugin.
Download Add to Search Bar from Mozilla Add-ons