If you cannot see the preview of the customized marker in your Firefox browser, it is very likely that you can solve this by adjusting the browser settings.
To do so please open Firefox and go to Options -> Applications. Set the "Portable Document Format (PDF)" setting to "Preview in Firefox".
Now you should be able to see the marker preview in the vanillNAV Manager custom marker designer.
If the preview is working, the marker preview is diaplyed at the right side of the marker designer:
Tags: browser configuration