Repeatedly getting the - “Firefox is already running, but is not responding" error message....

Hi I am repeatedly getting the; “Firefox is already running, but is not responding. The old Firefox process must be closed to open a new window” error message following a much earlier [fatal] FF crash. I have tried multiple deletions, Registry clear-outs & re-installs etc.– followed the several different & various "fixes" & suggestions on various forums etc. Have finally found 2 files in the profiles folder >Appdata>Roaming>Mozilla.Firefox>Profiles>*****.default that I can’t get to delete (have managed to remove all other files & folders in this folder thread but these two; “prefs” – a Javascript File “search.json.mozlz4” Mozlz4 file The error message on these 2 files when trying to delete is; “You require permission from Desktop-F71TMLM\”user” to make changes to this file”

I am logged in as Administrator, have checked the “Security” properties on each file and all the settings, permissions etc. appear to be in place supposedly allowing me to make changes/edit/delete each file – but it’s not happening. [And I tried ‘dragging’ them to another folder too – no dice] Suggestions please??