Open your tasks manager (either clicking right mouse button on the taskbar or use CTRL+Shift+Escape).
Go to Processes tab.
Look for all processes that are named “firefox.exe” and terminate them by clicking the “End Process” button.
Confirm when you see any confirmation message box.
Now try to run you firefox again (just please make sure you don’t click on firefox twice to run it just once).
You are receiving that error because firefox detected another instance of firefox hanging at background and cannot either close nor start successfully. that usually happens because errors in plugins, slow CPU, or RAM exhausted.