“A copy of Firefox is already open. Only one copy,etc.” * hour w* Apple support no help...

I spent many hours on my Mac following all of the 'solutions' online from Firefox and Apple. I finally called Apple support and it took over * hours on the phone with them. Mine was a particularly hard case. The solution was to go into depths of my computer that I had no idea existed and was surprised how many, many places the Mozilla and Firefox pieces were still there. Way too many to list even if I could remember. I had completely, I thought, removed every bit of Firefox from my computer. It is insidious and only when every possible kernel was deleted could I re-install Firefox. There were many, many 'specific' places to go to find them. My advice would be to find someone who knows your particular type of computer like the back of their hand and let them do it. Or call your computer's support team. It could take ** minutes or you may have to escalate to Senior Advisors and take much longer. Is using Firefox worth it? If I had known the frustration I would go through to get it 'fixed', I probably would have abandoned the idea of using Firefox early on, even though I have used it forever and really like it. Good Luck.