They are using CSS to make the background colour of your selection invisible. You can still select the text; it just looks like you can't.

    ::selection {
        background: auto
    }
    ::-moz-selection {
        background: auto
    }

See https://css-tricks.com/almanac/selectors/s/selection/

Just had the same issue. You can select it, but can't see that you've selected it. Probably some weird CSS interaction.

I didn't have a problem with it in chrome.