I know there have been some reports of this here and some discussion, but my quick search didn't find any threads on the subject.
I find this happening frequently in the last week or two, but I've seen it happen many other times going back much longer. I do a cold start on my laptop and, without one click or keystroke, there's Firefox....starting as though I had asked it to.
A good bet would be that it has something to do with
this. Except that it's only supposed to happen if Firefox was still open at the prior shutdown. Never do I leave Firefox or anything else open on shutdown. So it is over-zealous in operation for reasons unknown, but there are instructions for disabling this "feature" a little ways down the page. I've done that and will report back with the result.
Has anyone else been experiencing this?