I use Greasemonkey, Firemacs, Menu Wizard, HttpsEverywhere...and their relevance is mostly in that order.
Generally, I think a business built on building browser extensions is roughly equivalent to the shareware businesses of the previous millenium. If the extension is really useful and either solves a serious business problem or has massive adoption then a little money will trickle in. But by and large the problem is that the anchor price for the browser and browser extensions is $0. This means that $10 is infinitely more money.
And the occasional $10 is very very rarely enough to really run a software business and provide support and drive development forward. It's also not enough to support significant marketing and even significant marketing is unlikely to be enough to cut through the noise of the 'extension stores'.
Finally, I've been thinking that a lot of the real problem is that while $10 or $20 seems reasonable, the aggregate logic of cheap utilities is that if a person pays for all of them it is real money...e.g. twenty ten dollar utilities is becomes a non-trivial software purchase. I think people act on this intuitively, they're not going to pay for all of them and that means not paying for some of them and not paying for some of them is morally more or less equivalent to not paying for any of them. And so they are disinclined to pay.
Again, it's not impossible to create a revenue stream from this sort of software, but it is unlikely to be enough to replace a full time job.
Good luck.