What OS? If windows, home or pro? If windows, are you running any security software besides MS Defender? What version of Firefox? i think a better move would be to install another copy of Firefox on the D drive and then use the import process or Sync to get your data into the D drive version of Firefox. You can also just clean up the C drive by moving files to the D drive. Another fun thing you could do is just use a free partition manager for Windows to give C more space if the partition is small and you have more room on the drive.