Firefox does not respect aria-labelledby in elements named from content

The link in the following code snippet has an empty accessible name, despite both the `img` and `svg` elements having an accessible name:

```html

I tested this with buttons too, and it has the same problem. I suspect this will hold for all named from content elements (needs to be confirmed). This is such a substantial problem I think we should consider adding a check or rule to fail this.

Filed a ticket with Mozilla for it here: https://bugzilla.mozilla.org/show_bug.cgi?id=1652712

Start with the provided HTML reproduction and the linked Mozilla Bugzilla ticket, confirming the empty accessible name in Firefox for links and buttons. Determine whether the behavior affects other elements named from content, then locate the relevant axe-core checks and tests; completion should include a confirmed scope and regression coverage for the intended diagnostic.

Written by the indexing model from the issue text.