if your manual changes don't 'stick' after a restart of the browser, please look out for any suspicious extensions (like 3rd-party-toolbars or things you have not installed intentionally) presently installed under firefox/tools > addons > extensions or the programs section of the windows control panel and remove those entries. you also may want to try AdwCleaner in order to find & remove malware.

in case this doesn't solve the issue please try this: go to firefox > help > troubleshooting information > profile folder - "show folder", then a new window will open up. in this window look out for a file named user.js (it might be used to overwrite your custom settings). in case it is present , delete or rename this file and restart firefox afterwards.

Remove a toolbar that has taken over your Firefox search or home page

How to fix preferences that won't save

I have the same problem, it's caused by the Yahoo Toolbar Mozilla Add-On.

The fix is to restore Firefox to its default by going to Help===>Troubleshooting Information and clicking on the Reset button on the top right hand corner