Was it just the program files that were removed?

https://support.mozilla.org/en-US/kb/how-run-firefox-when-profile-missing-inaccessible

http://kb.mozillazine.org/Profile_folder_-_Firefox#Navigating_to_the_profile_folder

https://support.mozilla.org/en-US/kb/profile-manager-create-and-remove-firefox-profiles

https://support.mozilla.org/en-US/kb/back-and-restore-information-firefox-profiles

Type about:profiles<enter> in the address bar. How many profiles are listed? How many should be there? Also, open the folder in your file explorer.

Everything was removed. An older profile remained, but it did not contain my favorites or history. I want to know what caused this problem so that it never happens again. I have re-installed Firefox, but it has no history or favorites. How did this happen? Why was Firefox suddenly wiped off my computer? What will Mozilla do to permanently fix the problem? A user should always be notified and asked permission before such a major change occurs without warning or permission. MAJOR PROBLEM - PLEASE RESPOND.

There are several possibilities including malware and your anti-virus. While an installer could remove files/folders, I don't see it as likely.

Open your trash/recycle folder. Anything there?

You may have ad/mal-ware. 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. Each works differently. If one program misses something, another may pick it up.

Open your anti-virus history. Anything there?

I scanned with two anti-virus programs. There was no malware, nothing above level 2 out of 10 with 10 being the worst.

And you opened the profiles folder with your file explorer?

I called for more help.