I’m very surprised that many people are questioning whether developers should even be allowed to charge for add-ons. Traditionally it is true that add-ons have been freely available, but I’ve known of pay for add-ons for at least 2 years now (that add-on is still available so must be doing ok) and I imagine there have been some around for longer. Some companys’ entire business rests on their Firefox add-ons. That sort of situation wouldn’t exist if no-one ever paid for add-ons in some way.
I believe that developers should feel welcome to do whatever they please. If they believe their work is good enough to charge for and users think it is good enough to pay for then great. There are probably more funded add-ons out there than most people realise. Charging for download is just one mechanism for recouping some cash for development. I use adverts on my website to help fund some of my development. Some developers ask for donations, others are supported by virtue of their add-on driving traffic to their website. Do you think Google give you the Google Toolbar for free purely out of the kindness of their hearts?
I’m no expert in the matter, but I suspect there would be something seriously wrong with the add-ons ecosystem if it was only populated by free items. I’m pretty sure that people actually getting paid for their work will feel more driven to release higher quality add-ons as well as put more pressure on Mozilla to fix the issues that they find in the platform during their development. This helps all developers, not just those charging their users.
I suspect that people who don’t like the idea of add-on developers making some money out of their add-ons simply don’t fully understand just how much work developing add-ons involves. It takes lots of time and effort to get something to the level of quality you are happy with releasing. Those that aren’t developing for a company are literally taking weeks and months of time away from their friends and family to provide users with something useful. You can argue that if they enjoy the work then they shouldn’t be charging for it, I enjoy my day job yet I still expect to get paid. Believe me that working through all the little bugs your users find but often you can’t see is generally not much fun compared to the actual development of a new feature. Money provides that little extra incentive that might mean the difference between an add-on continuing to be maintained and it dying out.
If you don’t like that a developer is charging for their work, even if the same add-on was previously available for free then I don’t really think you have any right to complain. If you don’t think that what they are offering is worth the price then don’t buy it. I wonder, if you were to try disabling all the add-ons you are using for a day, how many of them would you so sorely miss that you could see yourself paying a small price for them.