Note that it is never safe to use a shortcut like Ctrl+Shift that only consists of modifier key to rotate through choices. Firefox has a lot of shortcut keys that work with modifiers and best is to assign a separate shortcut to each language. Ctrl+Shift would normally only have effect if you move the mouse with the left mouse button pressed.

Note that you can change the bidi text direction in Firefox via the Ctrl+Shift+X shortcut combination.