Firefox on its own will not change your default search engine. In most cases Firefox's default search is Google. If this has changed to another search engine without you making the change then it is possible that unwanted software on your computer is doing it without permission. The fact that you say the default keeps reverting to bing after you change it back to google makes me think that some malware may be high jacking your firefox. Please see the support article below: [[Remove a toolbar that has taken over your Firefox search or home page]] If you can identify the software causing the issue you should uninstall it from your computer and remove any related add-ons from Firefox. I also suggest you try these free programs to scan for malware, which work with your existing antivirus software: * [http://www.microsoft.com/security/scanner/default.aspx Microsoft Safety Scanner] * [http://www.malwarebytes.org/products/malwarebytes_free/ MalwareBytes' Anti-Malware] * [http://support.kaspersky.com/faq/?qid=208283363 TDSSKiller - AntiRootkit Utility] * [http://www.surfright.nl/en/hitmanpro/ Hitman Pro] * [http://www.eset.com/us/online-scanner/ ESET Online Scanner] I hope that helps. Let me know if not.

Use the search reset tool. * https://addons.mozilla.org/en-us/firefox/addon/searchreset/ And delete a possible user.js file. * https://support.mozilla.org/en-US/kb/how-to-fix-preferences-wont-save#w_user-js-file-overriding-preferences

An extension called ''Bing something'' had been installed without my knowing and was listed with the other regular ones. I wish I had thought about looking in the extensions category or that you had suggested it to me. Anyway, the pb is solved.