cannot close windows nor end process after update to firefox 38

Firefox 38 on Ubuntu 15.04: after update, I cannot quit Firefox, the process(es) keep running in the background. I have to 'killall' to end the processes. I can no longer close Firefox windows (they are unresponsive). Each time I start Firefox (after a killall), it spawns a *new* window. I am up to six open windows, none of which I can close. This has been going on for about two days, despite multiple system shutdowns. Also, the search function above the tiles when I open a new tab is unresponsive (the search next to my address bar still works, however). Firefox is unusable in this state.