I made a couple browser extensions that make over $500/month each. The key seems to be naming your extension after high-volume search terms and getting good reviews on the chrome store (and obviously having an extension that works well and solve a common problem on major websites).

I'm extremely biased (see below) so take my opinion with a grain of salt, but I think browser extensions are a pretty neat way to break into indie hacking. They cost nothing to run because they're hosted by extension stores. They're often faster and easier to build than whole apps because you can just use them to fix or modify existing websites rather than create your own from scratch. They can get organic traffic from extension stores, especially if they're well-named.

The main piece that was a pain in the ass for me was adding payments, so I made a service to do it (https://extensionpay.com), and now I can just focus on making the extensions work well. Because of all my previous work I was able to build and submit my last extension (making over $500/month now) to the chrome store in four hours — no joke! It was a really cool moment. Plus, running an extension monetization API I'm able to see all the extension that make real money and learn from what works for them.