Changes the selected search engine in the search bar by scrolling the mouse. This saves you from having to click the menu, and then click the engine you want to use. This works both when you're hovering over the search bar, or when you've selected a word and right clicked to search the web for it, then you can scroll when you're hovering over the "Search google for..." menuitem. The add-on also supports keyboard shortcuts ctrl+k and ctrl+shift+k to change the search engine with the keyboard. Optionally you can select to scroll in a loop by configuring it in the options window of the add-on.

(Original extension is at https://addons.mozilla.org/en-US/firefox/addon/3696)
(I just adapted this extension to work with Songbird.)