It sounds like you have a third party program that has taken over your search engine, home page, and/or the default new tab page. Fortunately, this can be remedied easily:

1. At the top of the Firefox window, click on the Firefox button (Tools menu in Windows XP), and then click Add-ons. The Add-ons Manager tab will open.
2. In the Add-ons Manager tab, select the Extensions panel.
3. Select the toolbar you wish to remove.
4. Click the remove button.
5. Click "Restart now" if it pops up. Your tabs will be saved and restored after the restart.

After Firefox restarts, install the Search Reset Tool. This will remove the rest of the traces of this program from your Firefox.

For further information, please read Remove a toolbar that has taken over your Firefox search or home page.

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For more

• http://removal-guides.com/malware/conduit-search-hijacker/

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. also run a full scan of your system with different security tools like the free version of malwarebytes and adwcleaner.

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