Firefox just locks up. The mouse cursor can be used to resize the window, but is otherwise unresponsive, except to to minimize, maximize or close the window. If I click minimize and then restore the window, Firefox will have executed the last mouse-click before minimizing. This happens after five minutes or so after starting Firefox, whether I have only 4 or 5 tabs open, or a whole pile of tabs. All I can do is click X in top RHS, and then restart Firefox. Then click the "recently closed tabs" to get back to where I was. I have tried clearing the cache and (most of) the cookies. As well I did a '''complete refresh of Firefox, and disabled all the add-ons''', but the problem is still there Running on a (old) Lenovo Thinkpad E545 with 16GB, Ubuntu in a SSD using a 50 GB partn with 30GB free. This problem occurs '''only in Firefox'''- all other apps continue to run!