Background on my configrations:
I have a shortcut key of "Command + ," set to Preview the Primary browser (Chrome)
I have a shortcut key of "Shift + Command + ," set to Preview the Secondary browser (Firefox)
I just had the problem and fixed it with the following steps:
1) I switched Primary browser to Firefox
2) I switched Secondary browser to Chrome
3) I switched them back and now it works like it always did