Plugins for various things - browsers (Firefox / Chrome extensions), apps (Figma), and platform providers (Salesforce, Shopify, etc). Find a feature that is needed but not offered, build a plugin for it, sell. Some of these have built-in monetization infrastructure so you don't need to roll your own.
This is probably an underserved area, actually. There are a lot of patterns of behavior we each have which involve our browsers, and there's probably room for some usage optimization extensions.
Do you know of a good site that focuses on browser extension reviews and discussions? I now realize I could probably find things which would improve my life, but I don't feel like doing a random walk through the general extension page of Firefox.
