If it gets changed to the Yahoo Serach Engine even though you edited the search engines it is eiter an add-on that causes the trouble or it is malware on your computer.

1. Try booting Firefox in safe mode to check if an add-on is causing the problem
2. Run a malware programm or check you application list in the Windows settings to see if malware is installed

Hi j256, I think the first reply is good advice, but let me expand on that.

Don't bother trying to remove the built-in Yahoo search plugin; instead, focus on your default search engine setting, as described in this article: Change your default search settings in Firefox. Firefox is designed to allow you to change your default search engine once and remember it permanently.

If that changes without your doing anything, the cause might be an extension, locked settings files, or external software.

To address those possibilities, here's my suggested procedure for tracking down and cleaning up bad add-ons and other hijackers. I know it seems long, but it's really not that bad.

(For Windows Vista, 7, 8, 8.1, 10; XP is somewhat different)

(1) Open the Windows Control Panel, Uninstall a Program. After the list loads, click the "Installed on" column heading to group the infections, I mean, additions, by date. This can help in smoking out undisclosed bundle items that snuck in with some software you agreed to install. Be suspicious of everything you do not recognize/remember, as malware often uses important or innocent sounding names to discourage you from removing it. Take out as much trash as possible here.

(2) Open Firefox's Add-ons page using either:

• Ctrl+Shift+a
• "3-bar" menu button (or Tools menu) > Add-ons
• in the Windows "Run" dialog, type or paste
  firefox.exe "about:addons"

In the left column, click Plugins. Set nonessential and unrecognized plugins to "Never Activate".

In the left column, click Extensions. Then, if in doubt, disable (or Remove, if possible) unrecognized and unwanted extensions. All extensions are optional; none come with Firefox.

Often a link will appear above at least one disabled extension to restart Firefox. You can complete your work on the tab and click one of the links as the last step.

Any improvement?

(3) You can search for remaining issues with the scanning/cleaning tools listed in our support article: Troubleshoot Firefox issues caused by malware. These on-demand scanners are free and take considerable time to run. If they finish quickly and especially if they require payment, you may have a serious infection. I suggest the specialized forums listed in the article in that case.