I understand that when you have two windows next to one another that they randomly switch between the two when you copy data from one to the next.

Does this happen in Safe Mode? Diagnose Firefox issues using Troubleshoot Mode Does this happen in version 33?

Are that windows in the same Firefox instance or do you have multiple Firefox instances open with separate profiles by using the -no-remote command line switch?

• http://kb.mozillazine.org/Opening_a_new_instance_of_Firefox_with_another_profile

Note that it is possible that a window takes focus if there are plugin-container processes active that try to take the focus.

• https://support.mozilla.org/kb/Troubleshooting+plugins