—This means that although you ‘think’ Firefox is closed; it isn’t. It’s still residing in memory and taking cycles - but isn’t able to be displayed because you’ve allegedly ‘closed it’.—
So, I’m not the only one. I’m just posting to vote for your response. It is what causes the problem for me on the rare occasion I have it. And it isn’t just Firefox. Winamp and some other software does it as well. I just get in the habbit, when an application won’t run, of checking to see if windows thinks it’s already running. I delete the one that shows up on the list and life is good again.
Worked R&D in my past life; did lots of validation runs to debug firmware and RTL code in numerous OS’s. While I didn’t see this happen with WinAmp; I have no doubt whatsoever that this may have been the case. I just noticed this with Firefox more than any other application that I commonly used.
I didn’t have this issue with Chrome, or any of the various other applications I have used. So, no; you are far from the only one who noticed this issue with Firefox.