The
HTML panel displays a live view, i.e. the contents as they are interpreted by the browser, not a source view.
Firefox automatically corrects HTML entities that are missing a semicolon and doesn't provide any info that the entity was incorrectly defined.
So to find such errors, you'll need to search within your IDE. You may also want to
create an enhancement request at Mozilla, so that they they throw an error message in such cases.
Sebastian