Hi   !

Go to the 3-bar menu => Addons => Extensions and look for any unfamiliar or suspicious looking extensions, like e.g.:

SafeSearch Incognito SearchAssist Incognito

When you find any :   remove them.

Also look in : 3-bar menu (or 'Tools') => Options => Advanced => Network tab => Under : 'Offline Web Content and User Data'

Do the same in Windows' Control Panel => Programs.

However :   most malware will be 'hiding' somewhere , so let's run malware scans :

Further information can be found in this article :
https://support.mozilla.org/en-US/kb/troubleshoot-firefox-issues-caused-malware?cache=no

Run most or all of the listed malware scanners.
They all work differently   -   what one program doesn't pick up, the other might.

Also see : https://www.im-infected.com/adware/remove-online-form-finder-toolbar.html

You can find the install location of a specific extension in the extensions.json file in the profile folder. You can open the extensions.json file in the Scratchpad and search for this extension. The descriptor field should list its install location.

• "3-bar" menu button or Tools -> Web Developer
• click the "Pretty Print" button to format the file for readability.
• https://developer.mozilla.org/en/Tools/Scratchpad

You can use the button on the "Help -> Troubleshooting Information" (about:support) page to go to the current Firefox profile folder or use the about:profiles page.

• Help -> Troubleshooting Information -> Profile Directory:
  Windows: Show Folder; Linux: Open Directory; Mac: Show in Finder
• http://kb.mozillazine.org/Profile_folder_-_Firefox