Beyond the defaults that Windows O/S and Firefox uses for their windows version hotkeys - changes beyond that is through 3rd party apps or userChrome.css edits to reassigned keyboard commands.

If you mean sequential navigation, i.e., Alt + Right Arrow goes Next, Next, Next along the tab bar, you can use this add-on I created for someone at some point:

https://addons.mozilla.org/firefox/addon/hotkeys-for-tabs-left-right/

(That doesn't eliminate the standard Windows keyboard shortcut. Add-ons aren't allowed to directly change or remove built-in keyboard shortcuts, the only method is to inject a script into each page to intercept the shortcut, and it doesn't always work.)

Oh wait, you want Alt + number. There is a different add-on for that:

https://addons.mozilla.org/firefox/addon/alt-number-to-switch-tabs/