For Windows 10 OS, Do I need MFF "x86" ?

There is a problem with that, it might uninstall both. and take everything. Are both showing the same bookmarks and saved other stuff ? If so then should save your profile in the 64bit one. https://support.mozilla.org/en-US/kb/profiles-where-firefox-stores-user-data Then uninstall both Then Delete the Mozilla Firefox Folders in C:\Program Files and C:\Program Files(x86) Then restart system. Then run Windows Disk Cleanup. Then run it again and click the button that says Cleanup System Files. Note: your Firefox Profile is saved. https://fileforum.betanews.com/detail/Mozilla-Firefox-for-Windows/1032985422/1 Firefox 57 released this date early. Will be on Mozilla Servers tomorrow. This site have used 20yrs so is fine. Note : Legacy Extensions will be disabled and or removed. If need help with your profile come back Please let us know if this solved your issue or if need further assistance.