Well, if you have already lost your profile, try to delete entirely Firefox from your computer : https://support.mozilla.org/en-US/kb/uninstall-firefox-from-your-computer (or search "delete entirely Firefox" on the web).

After that, check your firewall, your antivirus and your antimalware. It is time to make a security check-up of your computer, to be sure it is clean.

Then download again Firefox, from the official website, and try again to see if the problem is still here.

This can be caused by a problem with the profiles.ini file.

It is probably best to delete the profiles.ini file to force Firefox to create a new default profile

• http://kb.mozillazine.org/profiles.ini_file

You can use this button to go to the currently used Firefox profile folder:

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

• Windows: %AppData%\Mozilla\Firefox\
  C:\Users\<user>\AppData\Roaming\Mozilla\Firefox\Profiles\<profile>\
• Linux: ~/.mozilla/firefox/
• Mac: ~/Library/Application Support/Firefox/

• http://kb.mozillazine.org/Profile_Manager
• https://support.mozilla.org/kb/Managing+profiles