I'm using FF 59.0.2 (64bit) under the Mate desktop in Ubuntu 16.04. I'm using Tree Tabs. I generally have about 300 tabs open, across about 9 FF windows. For the last few months, I've found that several times through the day, if I go to use FF, it has become unresponsive: - If I click on a tab, nothing happens except the title on that Window updates to the title of the tab. - The tab does not gain focus - The contents of the tab are not drawn - If I change focus to some other window, the FF window will partially update - I cannot reliably click on any HTML controls (buttons etc.) which appear after doing the focus change to get some content to appear - The FF buttons and menus stop working - FF will stop working after a random period; I can be actively working in other windows, then change to FF and discover it has stopped responding to mouse interactions - I have to kill the FF process and restart it. - After choosing the option to reload the session (with the apology about having trouble restoring it, every time), FF works normally for a while (sometimes, for 2 or 3 hrs).