It’s the right of developer to sell an extension or not. If he does, he should remember the following consequences:
- If it was for free, users will try to live with the existing version and keep it alive = continued development on their own (at least private) . The user rate will drop by more than 90 percent, iirc only three percent of the internet users is willing to pay for things in the internet [citation needed].
- External supporters like testers and localizers will often loose interest in supporting your product (because they think that they already invested something in your product and won’t pay anything for it).
- Clones of your product will probably develop. If they already existed, people will replace your add-on with theirs, even if it has less quality.
- Indirect payments by the users should be tested: Adding ads to your add-on or the web pages or changing the search engine could be an alternative.

Consequences for AMO:
- If a free version had a license which allowed to use the code, the files shouldn’t be deleteable.
- There should be an easy way to exclude buyable extensions from search results (just visit a page with freeware and shareware and you will get know how frustating it will be if you can’t). Free limited versions which only describe the limited versions are ok to list.
- Maybe paid versions should only be linked from AMO (there is a general bug about external add-ons and AMO), so they can improve their quality by submitting fixes as updates faster to the consumers (without the need for amo editors). Also, paid add-ons are often more complex and so the chance to get reviewed is lower.

If you don’t do that (differentiate them and prioritize free over paid) and most of the people working for free will switch to ask money for their work or even stop working, and that would be really bad for Mozilla.

Examples of commercial add-ons: Attensa Feed Reader, iMacros, Loop-to-PDF and many ohers.

Mozilla Inc. shouldn’t support this.