Firefox was upgraded to 38.4 ESR automatically, while I have to say the 38.4 is a nightmare for me. It will crash several times every day, even I just open one or two pages!
Maybe there are other causes for its crash, but I encountered such issue on both different laptops with different OS, one Win7 and one RHEL, so I guess it should be caused by the new version.
Update: I uninstalled the FF, removed all the files related with it, then install the 38.4 again. Hope this issue could be fixed!
Update: this issue did be fixed by the above action.