The problem is that consistently I've been seeing the browser hang up after 20minutes or more. In the bug that I filed at Mozilla's Bugzilla (https://bugzilla.mozilla.org/show_bug.cgi?id=655957) I mentioned a few sites but eventually it will happen on any site as far as I can tell. Even on Mozillazine's forum.
First I thought it was a Fx issue with Flash since that seemed to be the only correlation I could think of but after some extensive and annoying testing I've successfully identified NoScript as the "cause". Since it's working with older versions of Nightly without any problems I guess some checkin on mozilla-central in those 15 days caused NoScript to behave erratically. Unfortunately I can't further investigate the issue due to Bug 652747 but I thought I'd mention it anyway. The bug I filed on Mozilla's Bugzilla also contains some WinDbg64 stacktraces if those are any help at all.
Hope you guys can find a solution
