Right Click Context Menu on Ubuntu Firefox Not Working

For the last few months in Firefox on Ubuntu at times the right-click that brings of the context menu would not work. More specifically, when I right-click on a link (e.g. to open in a new tab) the context menu pops up but any movement of the mouse results in the menu immediately disappearing. Very annoying. The first solution was to simply use the keyboard arrows to navigate the context menu, i.e. right-click then press down arrow and I could select the desired menu option. But still quite convenient. Another problem is that in the address (location) bar auto-complete stops working. Normally, typing a letter or two in the address bar brings up the list of recent URLs visit starting with those letters. Pressing enter selects the most recent entry for example. Well this also stopped working when the right-click problem occurred. Both of these problems have been on different versions of Ubuntu - currently 11.10, prevously 11.04 and possibly, but not sure 10.10. Most of the suggested solutions initially refer to add-ons. Its an add-on causing the problem; disable them one-by-one to find the cuplrit. Maybe, but (a) I'm too lazy to test (i.e. run without an add-on until the problem arises) and (b) I want to use the add-ons I have. I only use 4: Ubuntu Firefox Modifications, Global Menu Bar Integration (for Unity), Adblock Plus and NoScript. Surely they are popular enough that if they did have a bug it would be fixed by now ...? So finally I found a workaround. If the problem arises press 'Ctrl-N' to open a new Firefox window and then close that new window. The old, original window is back working again, with autocomplete in the address bar and right-click context menu. When time permits I'll search for the culprit add-on and a real solution, but for now Ctrl-N.