I keep having this issue in all browsers, in all my games since about January, probably around when I upgraded from Unity 2.5 to 2.6. The games play just fine, but when you close the browser tab they will wreck the browser. I really have no idea why this happens, it even happens with earlier projects or brand new ones.
See examples here (of course, be warned):
http://nihilocrat.110mb.com/unity/paincube.html
http://nihilocrat.110mb.com/unity/bombing_run.html
http://nihilocrat.110mb.com/unity/kreuzzug.html
This is an older project that I worked on pre-2.6 that also crashes:
http://nihilocrat.110mb.com/unity/verteidiger.html
I’ve had this issue for awhile and I’m really sorry I haven’t got a single clue or error messages, it’s kind of what kept me from mentioning it until now.