Versions tested on 11/04/2024

Firefox 132.0.1
Chrome 130.0.6723.92
Edge 130.0.2849.68

11/04/2024

Firefox On Windows: To open the select input, alt+downarrow, and then use the arrow keys to navigate through the options.
If you're focused on the select input, you can just use the arrow keys to select them.

On MacOs: To open the select input, option+downarrow, and then use the arrow keys to navigate through the options. On MacOS, after opening the select input field, arrow keys don't seem to work. Also, if you focused, using the up/down arrow keys seem to just open it...

Firefox bug report -

Chrome/Edge On Windows: To open the select input, press 'enter', and then use the arrow keys to navigate through the options.
If you're focused on the select input, you can just use the arrow keys to navigate/select them.

On MacOS: To open the select input, just start using the arrow keys.