Assuming you’re the one adding HSTS, you’ll have to inspect the cert and/or view the content that is getting returned. On desktop Chromium you can type “thisisunsafe” to load a page even with HSTS. Not sure how to do it on mobile FF.
Would seem weird for it to be intercepting your domain’s traffic but not the rest of the internet.
Edit: just noticed you’re not even loading an SSL page. Are you using https in the URL?
Use a Chromium browser to inspect the cert.
If anyone knows how to get Firefox to show a bad cert before loading the page, I’d love to know.