• Uninstall Firefox (Programs and Features)

Then

Click Start, copy in search [B]%appdata%[/B] Then delete folder Mozilla
Click Start, copy in search [B]%LOCALAPPDATA%[/B] delete folder Mozilla

Then delete following folders:

C:\Program Files (x86)\mozilla firefox
C:\Program Files (x86)\Mozilla Maintenance Service

Restart your PC.
Then install Firefox again.

https://www.mozilla.org/en-US/firefox/new/