When you post a BBCode that you want to remain as text, you can enclose it in a
[code]
or
[icode]
BBCode so it doesn't get replaced.
Based on what I'm seeing here, there is something in your browser configuration that prevents Twitter's iframe from being loaded. I notice that there should be a small Twitter logo in the top right corner of the page that says "Loading tweet" and if that image is missing it's possible that the same thing that blocks iframes also blocks images, and subresources in general. Try a new Firefox profile or disable all of your Firefox extensions and you may find what it is. Otherwise, look into your browser console for error messages.
For reference, if you're using a recent version of XenForo and this add-on, that tweet should be embedded using this page:
https://s9e.github.io/iframe/2/twitter.min.html#1558479530322427904 - Does that page works on its own?