Search is a lucrative business on the Web and abusive add-ons and other downloadable apps like to hijack your search tools in Firefox so that they can make money off of you. Not only does this mean your searches might be routed to a less effective search site, but it may mean your searches are being tracked by unscrupulous people or companies.
If you're the victim of search hijacking, there's a one-click fix available from Mozilla.
Simply visit https://addons.mozilla.org/en-US/firefox/addon/searchreset/ and install the add-on. It will install, clean up your search settings, and uninstall itself all nearly instantaneously. You can walk away knowing that you've dealt a blow to the scammers and strengthened your privacy at the same time.
If you're not sure if you're the victim of search hijacking, go ahead and use the add-on. If your defaults haven't been changed, it won't do anything.
Thanks, Gavin Sharp!
Oh, and while you're at it, head over to the Firefox Plug-in Check to make sure your plug-ins are all up to date. If you're all up to date, take a look at that list and see if there are any plug-ins there you don't remember installing or using and disable those. To disable unwanted plug-ins, simply load up the Add-on Manager (from the Firefox menu on Windows 7 or the Tools menu on other platforms -- or by typing about:addons in the addressbar,) then click on the tab labeled Plugins. Once there, just click Disable on any plug-ins you don't recognize. (You can do the same for Extensions too.)
Think of it as spring cleaning. Your Firefox will feel a lot better after this minor tune-up.
update: be sure to un-check the "install some crapware too" checkbox on the Adobe Flash update site if you are updating your Flash because it was marked as insecure by the Firefox Plug-in Check page. Otherwise you'll probably end up with more search hijacking or other unwanted modifications to Firefox.