How do you earn from creating browser extensions?

You can sell extensions in the same way you'd sell digital downloads on a website. If you really think about it, browser extensions are just tiny little webapps that run in your browser, so people pay for things like productivity, security, anonymity, etc.

Examples:

https://www.tunnelbear.com/
https://www.ghostforchat.com/
https://www.joinhoney.com/
https://www.vysor.io/
https://www.gmass.co/
https://www.streak.com/

You have a choice of integrating with a particular app (like providing extra features to Gmail) or providing a basic service (like VPNs).

If you build a price comparison extension, there's another way to monetize: add affiliate links to your products. For instance, "This product is 15% cheaper on Amazon, click here!" And then you'd get paid a slice of their payment. With tens of thousands of users, this could add up quickly.

I think grammarly is a good example of an extension first app that makes money. https://www.grammarly.com/

I have a friend who is looking for a Chrome extension developer. This is one way to monetize it, do it for others for $$$.

The other way would be Streak's example. The extension is free, but there is a usage/feature limit above which your need to create a paid account. But this is more like turning the extension into a more serious business.

As for a multiple source income stream (the popular one here, that's why I bring this up), I am not sure...

I just put a donate button in my extensions. But this doesn't work well :P