The popular "Stylish" browser extension, which allows you to download custom styles for a given site (such as making text more readable, removing UI cruft, or adding a professional white background), has been silently logging its users web activity
for the last year and a half. The new owners of the add-on at the time, SimilarWeb, offer products including "Market Solutions To See All Your Competitors' Traffic."
While the data is supposedly anonymized, many URLs can be trivially de
-anonymized, especially when linked to your cookies on popular sites for downloadable styles, or even just from data in the pages you're visiting such as login or password reset tokens.
Mozilla has removed Stylish from its add-on catalog, but as of this writing it is still available on the Chrome store, and neither is likely to automatically disable or remove the add-on even for your protection.
Alternatives exist, including a fork of the pre-spyware Stylish called "Stylus" for Firefox
However, this is likely an important lesson about the security risks of custom browser extensions... even those originally developed with good intentions can be acquired by people with more sinister and/or incompetent security practices.