These questions will become more relevant with paid extensions, also for for Mozilla since they provide the "OS". The guy with url-fixer has a name to protect and polish so he will not have any interest in dumping support. Other developers you might only know from an email addy could have other motives. If shop must be safe for buyers content can only come from approved elite developers who has an interest in using Mozillas name for own purposes.
Not like Mozilla does not help developers already. Shop is only about $$$, the exciting math - imagine 1$ per Adblock Plus download or 10$ a year per subscriptioin to Filter list! From a commercial view point what are they waiting for?
Mozilla should copy Wordpress/Automattic. They encourage monetizing Wordpress, there are "commercial" advertisements at wordpress.org, but wisely stay out of that loop
Others can do what they want as long as licensing is not broken. Another issue for Mozilla shop I think. Really difficult to see how Mozilla can avoid getting in trouble. They have no history of being able to make clear politics regarding add-on site so in case of problems they will just go hmmmm for months and months.
ScribeFire made by Finke almost screams for a Pro version. Must be very easy to make a limited free one. Unique extension, not much more than Windows Live Writer as alternative. I don't think he understands marketplace if he believe url fixer is a good product. May be also why attempts to insert ads via Scribefire failed
People did not seem to like such monetary tactics - and now some don't like he throws in other extension as free "extra". Just because he is a good developer does not mean he is good at making money - but his problem.