Firefox 20 on Ubuntu 13.04 under Gnome 3 environment. Everything works fine, except this happens sometimes, not sure of the cause - The context menu works fine everywhere on all parts of Firefox, on all the bookmarks on my bookmarks toolbar, but for one specific set of bookmarks, namely the ones in the drop down arrow menu for the bookmarks that do not fit on the main toolbar, sometimes context menu for items on that appears entirely disabled (grayed out) though it works fine everywhere else including the bookmarks that are visible on the toolbar. A restart solves this but it happens again. This doesn't seem to be caused by an extension as I do not have any extensions that modify the bookmarks toolbar or manage bookmarks. Unfortunately, I use 15 other extensions for different purposes and disabling them one by one and checking everytime for one hour whether the issue occurs again is not a viable solution. Please advise whether this is a firefox bug, or if an issue with extension framework, then how to single out the faulty extension. Thank you.