Did you try to close and restart Firefox or reboot the computer ?

Note that it is usually best not to use a keyboard sequence that only used modifier keys like Ctrl+Shift or Alt+Shift to rotate the language (keyboard layout), but to set a specific keyboard sequence to avoid issues with (context) menu access keys in Firefox. Firefox uses a lot of commands with both Ctrl+Shift (Undo Closed Tab: Ctrl+Shift+T) and Alt+Shift (access keys on web pages) modifiers.

I've restarted firefox several times in that period and rebooted pc, it didn't work, but NOW it is fully working with ctrl+shift. I don't know how the problem is solved at this moment.

I'm curious, doesn't this shortcut cause issues if you use commands like "Ctrl+Shift+Delete" or "Ctrl+Shift+T" in Firefox (i.e. is there possibly a short delay to activate the layout switch) ?

when I type ctrl+shift and leave them pressed nothing changes until I release shift or add "t" or any other symbol (that can activate some action). And the answer is no, it doesn't cause any issues or delays.

The issue appeared again. Not changing language with ctrl+shift. ! But as soon as I close browser and open it again - everything came back to normal. ! I see that it is firefox problem (maybe from last update).

You can consider to assign a specific shortcut to each layout to select this layout like Ctrl+Shift+1.

It is possible that some status keys have the wrong state and Firefox or Windows sees them as still pressed.

Try to press all the status keys (Shift, Ctrl, Alt) a few times to see if that helps to recover from this. If it doesn't help then close and restart Firefox or reboot the computer.

I think there is more problems except language change. No sound coming out from any youtube/twich/fb/other. But at the same time in opera and on pc the sound is playing (windows player and some music in opera). As always closing browser and starting it again solves the problem, but it is not solution and annoys me. I think that it is a bad last update that caused it.