It’s probably not news, maybe it’s well known but it somehow was for me, having spotted it just yesterday night, and it might be for others, so here goes.
You can add it there by just going to any RB page except from the forums I guess, and clicking on the search widget in Firefox. There should be an “Add RB Search” option.
After that, you just type in whatever you want there, click on the RB icon and it will take you to the search results.
Eliminates the need to go to the rest of the site to search stuff when you’re browsing the forums at least. And Googling for results (though I still feel that Googling stuff on RB should work properly, new pages are NOT indexed properly by it and it hurts the site).