Your comments

Hi Vladyslav,

I believe you might have a pessimistic view of your own software, TO, which is actually very good and in many ways better than concurrent extensions that have more users. TO delivers already a lot of concrete features today, and has a big potential to be a true killer extension for anyone that use extensively internet for researching and reading stuffs about many things.

I personally would be happy to pay a small fee for it (5-10$?) if it was a paying extension (maybe a free + a paying one could also suit your case), and/or contribute to fix issues I find and improve with functionality I think are missing (such as cloud sync) if it would be open-source.

Concerning open-source, wikipedia has a very extensive list of possible business models which have proven to work for some projects:
You should also realize that even if you would open-source TO, it would still be possible to actually sell it. It may raise a few concerns:
- A user could always get it from the repository and package it himself before importing it in his browser. However this would be limited to power users only. Many people, even geeks who have the skills for it, would still prefer paying a reasonable fee and have the extension available in literally less than a minute with just a few clicks, rather than going through the hassle of actually doing the packaging (even more if it involves compilation) and install manually an extension in the browser. Also, people are respectful of open-source works, especially when the software they get in return can provide a service as amazing as TO can already do.
- Yes, there is always a risk of someone cannibalizing it to make it available for free, or worst to make it paying. However, if TO assert its position as the true source of the software from the beginning, and if TO embrace the spirit of open-source at large where people can actually improve the software directly, then it will be recognized as the original software by people and media. That's not something the copycats could have.
- Someone can fork the project and build a better extension. Yeah, that's one thing to accept in open-source. But it should also be understood that it happens only when there is a big clash of ideas within a community of developers. If you manage well your community, and make the contribution rules clear from the beginning, then you can avoid such clashes and forks. This is limited to your social skills, but good books already exist on it (such as The Art of Community from Jono Bacon, the former community manager of Ubuntu). Also, if you choose the right license, even if someone fork TO you would still be able to take their improvements and add them to your TO. Indeed, this work both ways, otherwise that would not be fair. Finally on this subject, complete open-source development models are based on forking: github is a prime exemple. Here, people fork massively to add the things they want in a project, and then propose them back (known as "pull request") to the original project which can accept the modifications or not.
- A concurrent extension can "steal" some code. Again, if you choose the right license you can avoid this (and if a concurrent still do steal code without contributing/open-sourcing back its modifications, then you know you can have the law to back you up). If a concurrent open-source extension does this in a way permitted by the licenses of both projects, then this is considered to be a good thing. Because once again, it can work both ways :).

About the choice of license, I would suggest you to avoid the ones which do not protect you enough in terms of "stealing" and forking. The MIT or BSD licenses for instance are very bad at protecting you from that.
As you are not making a library but a final software for end-user, even the most "extreme" licenses such as GPL would be acceptable, depending on your vision of how you want and think TO can develop itself in the future.
LGPL-style licenses are very often a good compromise.