Thanks for the update & upgrade! I did notice that the most recent FF (FF33 and FF34) seems to have some memory management issues - it crashes often on a 32bit machine with 20 tabs or more. And it crashes often when using the new Google Maps, which sucks BTW compared to the old one - seems to me the only purpose of the new Google Maps is to mess up FF as Chrome doesn't crash dealing with it.
With that said, another phenomenon I noticed was that when I close a tab (whichever way I close it, either DC on tab or DRC on page or TLC on page or Ctrl-W), occasionally half of the old closed tab's content shows on top of the newly-in-focus tab. It's a very weird phenomenon that happens on my Win8.1 64-bit machine(s). This is confirmed not due to DCCT because simply switching tabs by SLC (single left click) on a tab could lead to the same super-imposition problem. This happens when memory usage is high. So I suspect what you saw is a variant of this issue, and it might be due to FF rather than the websites. But this is just a hunch, I can't reliably reproduce the issue to file a bugzilla report.
Hmm, I've never seen this issue though. So if you double click on tab to close, it doesn't lag, but double right on page it lags? Did you notice if the lag is consistently on some web pages but not others? I suspect some pages might be doing something with mouse events, although that's still hard to explain how a page eventually end up being closed, just that the closing gets delayed.
If you can find a way to reproduce it - like it always happens on a certain page - send me URL. Also let me know which OS, FF versions are you using. I suspect it's OS related as FF behavior for right click/context menu on Linux and Mac was different than Windows before.