Hi TB. I'm Greg, an installation specialist, 10 years awarded Windows MVP, and Volunteer Moderator, here to help you.
- See if you can System Restore to a point before the problem began following this guide: https://www.windowscentral.com/how-use-system-r...
If the problem repeats then check which Windows Updates were installed at Settings > Update & Security > Windows Updates > Update History, uninstall them there or System Restore again, then immediately check for and block the Updates with the Hide Updates tool explained here: https://www.howtogeek.com/223864/how-to-uninsta...
- Check at Settings > System > Storage > Configure Storage Sense > Temporary Files > Files in Download folder to make sure it's not set to delete Downloads.
Another place to check is Disk Cleanup which may be automated by System Maintenance. Uncheck the box to delete Downloads folder: http://windows.microsoft.com/en-us/windows-10/d...
- Try Refreshing Firefox:
https://www.thewindowsclub.com/refresh-firefox-...
If that's not enough then uninstall Firefox in Settings > Apps > Apps & Features, restart PC, then reinstall Firefox from a fresh download:
https://www.mozilla.org/en-US/firefox/new/
I hope this helps. Feel free to ask back any questions and keep me posted. If you'll wait to rate whether my post resolved your problem, I will keep working with you until it's resolved.
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