Percula is an interesting...umm...monster.
I say that because it's built for the career site, Monster.com. After installing the extension, you access the extension through the context menu on a web page. Highlight the word or phrase you want to search on, right-click to get the context menu, and select "search on monster.com..." That's it. It will take you to Monster.com and search for the words you highlighted. Hmmmmm....
One of the things I've come to love about Firefox is the search engine box in the top right corner to the right of the address bar. You can add additional pre-built search engines by clicking on the icon and select "Manage Search Engines...", click on the link "get more search engines" and you'll be taken to the mycroft web site to select from a ton of already existing search engines. Firefox can even help you integrate other sites into Firefox's search engine box. There is even a Monster.com search engine at the site that you can add to your list.
Percula is good for quick searching, but I don't think it'll replace my search engine box at the top right. If the extension had more features, I would keep it, but installing Percula to search through Monster.com when I don't need the overhead is just asking for trouble, in my opinion.
Just my $0.02.