Change the key biding for theater mode from ALT+T to just T which is currently unused by any feature on basic twitch and keeps it in line with other shortcuts like F for Fullscreen or M for Mute. The reason is Firefox uses ALT+T to open the Tools tab in the files menu, the file menu tab is opened by pressing ALT while Firefox is the main window. To override this setting it requires manually disabling it by editing the configuration of Firefox or getting an unregulated extension from outside of twitch to edit it automatically.