Carolyn, I've been having this problem. When I updated to 10.9 I had about 20 safari windows open, many of them with lots of tabs. I tried your suggestion to delete
DefaultBrowser.plug-in from /Library/Internet Plug-Ins and then rebooted.
Before rebooting I had about 20 QTKitServer... not responding in Activity Monitor. Now, after startup and opening all the safari windows there are 10. They didn't show up all at once and usually (always?) showed up black and then going red in a minute or so. The good news is they all use < 0.1 CPU seconds before they die.
One thought I had is that I'm using the ClickToPlugin extension and than might be killing something that upsets QTKitServer. I also use Little Snitch to block unwanted outbound messages. I'll check both and report back....
Maybe I must delete something else? In /Library/Internet Plug-Ins I have
Here https://discussions.apple.com/thread/3245315 I read I must unstall 3ivx. It can help me? (I think it is not real help, because these topics was in 2011 year)