I'm not everyone, but many probably feel how I feel about browser extensions. The browser is free, and so are their extensions...

I'm fairly certain that upselling is the best approach to monetizing browser extensions... Provide a freemium service, and have paid features. Much like apps I guess.

But you really have to be providing a service. So it's more like the browser extension or app is just another marketing channel.

Lastpass is a good example. Distill Web Monitor is another. I believe the latter was acquired to be monetized.