On starting up Firefox, the window appears on my main monitor for a second before jumping over to my second monitor. It doesn't jump completely to the second monitor, just enough for it to disappear from the first screen. At first, I thought Firefox wouldn't start. I only discovered what was happening when I switched my second monitor on. I can still open a second and subsequent window normally. It is just the first one that does this. This a new problem and only started when I clicked on an email link to a webinar. I have tried restarting both Firefox and my Mac, but the problem persists.