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.

Thank you for trying! Windows Home, Norton security (with none of their extensions), Firefox 139.0.1, 64 bit.

 I use the Windows clean up & Norton Utilities to get rid of "system junk."

When I downloaded Firefox to the D drive, I remembered why it didn't work the last time- When I try to open it, there is an error message: "Firefox is already running, but is not responding. The old Firefox process must be closed to open a new window." No Firefox shows up anywhere in the Task Manager- there is nothing I can close. That's why I wondered if there is something I need to edit in the registry. I have not tried deleting Firefox from the C drive because I'm afraid of losing all my many bookmarks & passwords. Since I can't open the D drive version, I can't sync or import my data. I've moved everything I safely can (meaning that I know what the files are) to the D drive.

 Thanks again-

Without seeing your setup, I would just buy a bigger driver, get an external case, and then using free partition software copy you current c drive to the bigger new c drive.