I am using Firefox in Ubuntu for several years. Recently, after changing a monitor (or maybe after of one of several regular 14.05 LTS updates) I've found that Firefox freezes after hours/days to opened. It doesn't happen during the times I am continuously using but after I leave it for a while. I don't turn the computer of so Firefox remains always open. I have 6Gb of RAM (used 3.8/5.8) Resources are very low but Firefox has some activity: it is using 1.4 Gb Ram (several opened tabs) and System Monitor resources fluctuates between 5% and 6%. On the browser I click on tabs and the top bar displays the site title but it does not change to this tab. I clicked on the top right triple bar but nothing happened. Program menu is available, I can save the window with Session Manager but I am not able to load an old one. Other stuff like zoom in/out also doesn't work. I am able to File->Quit Opening Firefox again, session I had save with Session Manager is restored. The last tab contains 'about:preferences' which I had "opened" (not successfully) before saving the session during the freezzing. The only extension I am using (for a long time) are Session Manager, Tab Mix Plus and Ubuntu Modifications I have nor added plugins or extensions for a long time.