What would cause the ‘wp’ action to fire twice per page (but only once per post) in Firefox only?

Check the source for your page. I’m guessing that your source code includes a <link rel="next"> in the header, as WordPress injects by default, and Firefox is prefetching the “next” page in order to cache it.

Try this to remove that link from your header:

remove_action('wp_head', 'adjacent_posts_rel_link_wp_head');

Found this blog with more details: WordPress rel=”next” and Firefox Prefetching