Ask HN: Ideas on how to monetize Firefox or Chrome Extensions?

I'm looking to build out some ideas I have for FF and/or Chrome extensions. I'm looking for way to eventually build a business around these and I was wondering if there were any of you that knew of ways to monetize (reoccurring monthly payments, single purchase, or freemium is what I'm mostly looking at, though I'm open to others). I'm currently clueless on how to bill or set up "pro" versions of extensions.

Any links to where I could research more would also be of help. Also, any general advice about this space would be greatly appreciated.

Thanks!