Common revenue sources with browser-based games include advertising, in-app purchases, and subscriptions. I haven't seen a lot of merchandising, but if you can establish a good brand it could possibly work well.
However, another common approach you might consider is to sell licenses for your games or even develop them specifically for clients. It can take some work (and a bit of luck) to get established, but can be a great way to earn income if you're able to produce good quality work quickly. I believe this is a major (perhaps the major) income source for True Valhalla, who is quite open about his income and methods.