Try this: https://addons.mozilla.org/en-US/firefox/addon/ctrl-number-to-switch-tabs/

If it doesnt work, your best option is to either find another extension that does. or bite the bullet and configure a key remapper like keyd and make a "layer" to toggle the bind alt+(num) to ctrl+(num).

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.