Sure, if you lack your spine and moral compass. The business model is as follows:

1) Make a free add-on and have it gain popularity.

Then, either:

2a) Inject malicious code into it that delivers malware/advertisements, making you money.

or:

2b) Sell your add-on to a third party that will do 2a). If your add-on is popular enough, potential buyers will come to you themselves.

Other than that, no, I don't think you can monetize an add-on directly.