How long was the browser running when this happens? What were you doing at the time? surfing, watching a clip . . . .

Start Firefox in Safe Mode {web link}

A small dialog should appear. Click Start In Safe Mode (not Refresh). Is the problem still there?

Thanks for answering my post. Most of the time Firefox quits when I'm doing multiple things, like doing my taxes in TurboTax, answering iMessages and looking up things online via Firefox. Occasionally, I've left the computer temporarily, without closing anything, and have come back to find that Firefox quit. This last time, which is what prompted me to ask for help, I opened Firefox and it quit immediately.

I believe I tried starting in Safe Mode already but I'll do it again to make sure I did it correctly. Will let you know what happens.

Thanks again.