OK my Mac Pro is having sleeping problems again. This has been annoying me for more than a month now but I haven't had the opportunity to do any investigation. I have narrowed the problem down to being caused by Web browsers—specifically Safari and Firefox. If neither of those applications is running, the Mac goes to sleep by itself pretty much as it should. In short, the Mac won't go to sleep by itself if any of the following is true: Safari is open (with or without a window open) A Firefox window is open I have tried removing both the Flash plugin and the Click-to-Flash plugin (following Gruber's suggestions to do that) from /Library/Internet Plug-Ins and ~/Library/Internet Plug-Ins respectively. The Mac will go to sleep by itself if any of the following is true: Neither Safari nor Firefox is open (no windows, applications completely quit) Firefox is running (the application), but no Firefox window is open Can anyone please give me an idea for what I should be looking for to isolate this problem? The way I work it isn't really feasible to quit Safari (my preferred browser) every time I want the computer to sleep. I tried using the latest WebKit nightly builds, but they prevent sleep in the same way as Safari. I have several other plug-ins in the Internet Plug-Ins folders, including Adobe PDF viewer, Flip4Mac, iPhotoPhotocast, Java, nsIQTScriptablePlugin, Quartz Composer, QuickTime, SharePoint, Silverlight, and Wacom. I also use the slightly hacky Glims, but that hasn't caused me problems in the past and there haven't been any recent releases which might have triggered this problem. I'd be seriously grateful for any suggestions which anyone here can give. ___________________ Have I really flummoxed the great minds at MacRumors? Surely not! Seriously, guys—grateful for any ideas you can give to resolve this.