If I open a new browser window so that I have 2 up, for some reason I cannot open any new tabs in the second browser window. If I close the first browser window for whatever reason, the 2nd window will still not let me open new tabs, nor can I open any new tabs if I close all Firefox windows and open a new one. The only thing that seems to work is to open Firefox in safe mode and tick the box which puts the settings back to default. This gives me the ability to open new tabs in an original browser window, but again, as soon as I open another browser window I can no longer open new tabs. I also do not change any settings in between one Firefox restart and the next. I would very much like to know what is happening here, and if there is anything I can do to remedy it.