I have a 2010 Macbook Pro, and I just did a clean installation of the entire operating system because I was having these problems (and more) before. My Firefox will attempt to send a crash report after I close it (but says that for this crash, it cannot create a report), and then refuses to open until I restart the computer. I only get an error message that says that there is another copy of Firefox already open, and that there can already be one. I've already tried killing all firefox processes in the Terminal, but the computer then reports that there are no processes to be killed. When I restart, the browser opens up to pages that I had open maybe an hour ago. I doubt that a clean installation will help, as I just cleanly installed Firefox on a cleanly installed OS.