set Alt + Shift combination as keyboard layout switcher shortcut:

try if it can configured for a default app and check , or else use an add on that ll fullfill your need via FF

It's not Firefox specific. I have the problem in LibreOffice, too! I'll follow that in KDE communities.

Linux: Add Keyboard Shortcut to Switch App

First, install wmctrl.

wmctrl is a command line util to {switch, close, move, resize, set title, list, …} windows.

1. install wmctrl

sudo apt-get install wmctrl

Example use:

   wmctrl -xa firefox → switch to the window class name that contains “firefox”. (use this to switch apps.
   wmctrl -a firefox → switch to the window whose name contains “firefox”
   wmctrl -c firefox → Close a window whose name contains “firefox”
   wmctrl -l → list all windows

Type xprop in terminal to find the name of a app's window class name. [see Linux: Commands Related to GUI Apps])

here's some popular app's window class name:

   Firefox
   google-chrome-stable → Google Chrome browser
   gnome-terminal
   emacs

Now, create a keyboard shortcut to the command.

In your Linux desktop {Gnome, KDE, Ubuntu, Xfce, …}, there's usually a keyboard setting tool in the control panel to set a key to execute command. Use that.