Searchbastard is the most powerful Firefox Search extension I have ever used. I do not know how I got by without it before. Here is a brief description pulled from the Mozilla addons site:
"Quickly access your search engines and powerful "Searchbastard engines", which provides search options and can clean up the search results. Group engines, and perform multisearch. The "selection search" feature allows you to quickly look up words on a page, by selecting the text and typing the engine alias. The "adressfield search" extends the firefox feature, so you can also search the groups and searchbastard engines by typing the alias followed by the searchterm in the address field. The searchbar allows you to quickly perform a search, and quickly repeat that search using another engine. Everything is extremely customizable: You can arrange the frames in a multisearch, you can assign actions to all toolbar events, etc. The plugin is lightweight, and features are dynamically loaded when needed.
I use it in conjunction with Add to Search Bar or create a search plugin with one of the following sites:
Ready2Search Search Plugin Maker (Can Use Custom Favicons for Search Plugin)
SearchPlugins.net Search Plugin Maker
I also use a bookmarklet from https://www.squarefree.com/bookmarklets/forms.html to convert Search fields from post to get method when needed.
The ability to search multiple engines at one time, use different engines by simply clicking an icon, and create custom search groups make this a must have extension for me!!
Mozilla Addons Page