Originally I hated it because it didn't seem to work, but with excellent customer service (rapidly responsive to my inquiries, and provided suggestions that worked; namely refreshing Firefox (ver. 57.0) and using the built-in browser zoom), it seems to work well on all of the sites I visit, and remembers my zooming profile. There is an occasional burp (e.g., it shows an older version of the slider bar that doesn't work until it syncs something), but nothing I can't live with and nothing that's permanently broken. Considering that I was programming real-time simulations 37 years ago, I'm very satisfied with this add-on: it does what it purports to do, and the customer service is excellent. (Got more than I paid for.)