I haven't checked if there's a new Firefox, though I thought it updated automatically. But that's an obvious first solution, although it would be a problem if it was crashing on other people's PC's who haven't upgraded.
I haven't added any plugins. My game only has 1 (the Spaceship plugin for spaceship movement, which is fairly simple and has been in since day 1).
It does send auto-reports, so Mozilla will be receiving those.
I can't think what could cause it. It doesn't crash on other browsers, but what Darklinki says could be happening I guess. Since I always test in Chrome, if it's something that Chrome looks out for, I may have made a coding error that's causing an infinite loop.