Ghostery is a private Ad blocker that is used to block ads and stop trackers. When Ghostery add-on is used, the add-on blocks Marketo Forms.
To display Marketo forms on a Firefox browser with the Ghostery Add-on, change the Marketo URL in the form embed code to use your landing page CNAME instead.
The default embed code,
<script>MktoForms2.loadForm("//app-sjint.marketo.com", "123-ABS-557", 2019);</script>
should be Changed to use the CNAMEinstead of Marketo URL as below:
<script>MktoForms2.loadForm("//go.example.com", "123-ABC-557", 2019);</script>
Marketo forms embedded in this way may take longer to load as the form scripts are not fetched though the content delivery network. If you are embedding the form on an SSL secured page, you will need to have Secured Domains for Landing Pages set up on your Marketo instance, as there are issues with running an unsecured form on a secure page.