I used to have a pretty successful browser extension that I shut down because the site it worked on shut down.

I grew it by adding features that people would ask for on the site's forums, for example a user would say "hey can we have x feature?" and I would respond saying "great idea, I added x feature to my extension y!" This was very effective and over time others would start responding saying my extension had whatever feature they were asking, capitalizing on how relatively slow companies are to implement features.

This does rely on the extension having a site it operates on and having a forum for users though. If I were to do it today I'd focus on finding places where my extension's users concentrate, Discord, a community Slack, or otherwise, and doing the same thing.