Based on their privacy policy, it looks like they gather aggregate browsing data, which they may be monetizing. But they say that all data gathered is at the aggregate level.
https://addons.mozilla.org/en-US/firefox/addon/shoptimate/pr...
https://addons.mozilla.org/en-US/firefox/addon/shoptimate/pr...
The gathering of browser data in the privacy policy is to cover things taken by my Apache logs or click statistics (which I sometimes need to charge stores that pay me per click). But I don't monetize this in any shape or form.