Please uninstall Firefox. 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. Convert info to a MAC as I have no idea. Install Firefox 57 Quantum Full Version Installer https://fileforum.betanews.com/detail/Mozilla-Firefox-for-Mac-OS-X/1032985422/3 Site gets files day before goes up on Mozilla Servers, used site 20yrs. Note : Legacy Extensions will be disabled and or removed. That message is from the site and may indicate there is something unusual about your connection, for example, routing through a suspicious network or missing normal data (cookies, user agent, referring site, etc.). So https://support.mozilla.org/en-US/kb/troubleshoot-firefox-issues-using-safe-mode and https://support.mozilla.org/en-US/kb/troubleshoot-firefox-issues-caused-malware use more than 1 scanner, if need help : https://forums.malwarebytes.com/topic/9573-im-infected-what-do-i-do-now/ Please let us know if this solved your issue or if need further assistance.