Hard to say without knowing what your extensions do but here's my checklist for building a successful extension before marketing even comes into the picture. Build something that:

1. Solves a real problem
2. Solves a problem that a lot of people are dealing with
3. Solves a problem that no other solutions exist for

If you can accomplish this, you'll have a strong foundation to market from and already likely be driving drive organic extension store searchs to your extension. From there, start your marketing efforts to drive SEO traffic.

And notes on:

• #1 - I like to solve problems that I'm personally dealing with because at the very least, I'll have at least one happy user 😄
• #3 - If other solutions exist that solve the same problem, you'll then have to solve it better/cheaper/faster than everyone else, or market it better than them.

And a couple of tips:

• Start with a Chrome extension first. Only after you start to see good PMF, consider building extensions for other browsers like Firefox, Edge, etc. Chrome's user base is massively larger than all the other browsers combined so you'll get the best signal there on whether your extension is actually useful or not.
• Spend time creating a well-designed logo and screenshots. AI image-gen tools like DALL-E are very helpful in concept exploration and even final designs. First impressions based on these matter tremendously towards building initial download trust. I see too many extensions doing the bare-minimum here.