What type of issue is this?

Browser bug (a bug with a feature that may impact site compatibility)

What information was incorrect, unhelpful, or incomplete?

The article contains this (correct) info about Firefox compatibility:

It is worth noting that currently getSelection() doesn't work on the content of elements in Firefox. HTMLInputElement.setSelectionRange()) could be used to work around this.

However

What browsers does this problem apply to, if applicable?

Firefox

What did you expect to see?

Firefox compatibility issues should be noted in the browser compatibility section.

Did you test this? If so, how?

document.getSelection().toString() returns an empty string on input/textarea fields in my testing on Firefox 120.0.1

Can you link to any release notes, bugs, pull requests, or MDN pages related to this?

https://bugzilla.mozilla.org/show_bug.cgi?id=85686

Do you have anything more you want to share?

No response

MDN URL

https://developer.mozilla.org/en-US/docs/Web/API/Document/getSelection

MDN metadata