I have vimium which lets me set vim like key binds for firefox. It only works once the JS has loaded on a page so default screens like the new tab doesn't work. Other than that, it should let you rebind it to whatever you want, you can disable the other keys yoy don't need.

This article by firefox didn't help because the action/shortcut/hot key was not shown ...

I think it's just a preview feature, so it's not accessible through a normal menu yet I think, but if you type about:keyboard in the address bar and press Enter, I think it will allow you to remap these (under the Navigation section).

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:

1. A setting or issue with your desktop environment/keyboard layout
2. A keybind remapper you might've installed.
3. An extension you installed on Firefox

check if the alt/ctrl switch persists outside Firefox.