Firefox uses the hotkey CMD + SHIFT + E to switch between tab groups. This can not be changed. Boom disables the hotkey because it uses that shortcut for the visual feedback overlay. Even if you disable the overlay in settings it still blocks that hotkey. Very Frustrating.
You can change the Hotkey option for Boom 2 within the app. Please select the Hotkeys option on Boom 2 window as shown in the screenshot.
Next, tap on the options shown there and it will allow you to edit the Hotkeys.