Created attachment 20404 [details]
WebKit Plugin w/threading enabled and source code.
I do not get Xippee.plugin in the list of installed plug-ins after putting it into /Library/Internet Plug-Ins and restarting Safari. Of course, timer tests do not fail in this situation.
Would it be possible for you to make a complete buildable test plug-in for debugging?
Hmm, I got a crash with Xippee code in backtrace soon after removing the plug-in from /Library/Internet Plug-Ins, so I guess that It was loaded, after all. Anyway, I don't see timer failures. Is there anything else that needs to be done to reproduce the problem?