The initial items on the page (first ~25 items) are not AJAX but each subsequent load of activity is AJAX.
If this is happening in one browser but not another, this sounds like a browser configuration or browser cache settings rule. If you use CloudFlare, server configuration to set caching headers, or anything else, this may be causing issues with Firefox. If you really want to find this, you will want to simplify your configuration. If you are able to separate your testing to a test site on a different server or separate configuration, that may help you.
It could also even be a browser bug or oddity from Firefox (they have been having a lot lately).