http://iron9light.wordpress.com/2009/07/04/reset-your-firefox-location-bar-search-if-it-changed-by-avg/

I tried every suggestion that I could gleam from the net ..to get rid of the AVG search page...I tried forcing Google to take control by inserting the appropriate code into the home: config list....still no luck, until someone suggested I look for new( bold) strange looking entries in the config file and delete them....and bingo I finally had solved the problem. The real question is how did these lines of code get into the config list by themselves? AVG Aust/NZ have replied to me on this one, with some genuine concern at what happened to me..and it would seem that there may be some unintended reaction between the AVG tool bar code( I never installed the toolbar) & the latest version (5) of Firefox, notwithstanding, I had spent hours removing what I thought were all traces of AVG files hidden or otherwise from my PC. Yet the AVG search page still managed to over ride Google...an interesting question being: how were these rogue config code lines operating if I had deleted all normal traces of AVG code from my PC?

Found some the answer on AVG website, the rest on here.

First, download and save (don't run) AVG free from filehippo.com or similar. Re-boot your pc, locate the saved download of AVG and launch it. When it comes to custom install, untick/uncheck the boxes allowing AVG Safe Search and AVG Search toolbar to be installed. Now back up your bookmarks etc (this might help: https://support.mozilla.com/en-US/questions/857870#answer-226104 ) and then uninstall AVG and Firefox. Of course if you wish to keep AVG, don't uninstall it, just uninstall FF then re-install it. Use your profile back up (bookmarks etc) to get FF back to how it was. You may find you need to re-install add ons such as adblockerplus, themes etc afterwards though.