Sorry I realised I misread your post. It's strange your keybinds on Linux to change tabs are alt+num? For me, its Ctrl+num... and i think this is the default. The most likely reason is either:
- A setting or issue with your desktop environment/keyboard layout
- A keybind remapper you might've installed.
- An extension you installed on Firefox
check if the alt/ctrl switch persists outside Firefox.