Since Firefox 140, Mozilla requires every extension to declare what data it processes. "Website activity" is the closest category for what this add-on does: when you open a Steam store page, it sends the Steam app IDs shown on that page to my server (sub.aligueler.com) to check whether those games are in Game Pass, Ubisoft+ or EA Play. That's the entire purpose of the extension, without that lookup it can't show anything. Nothing else is sent: no Steam account, no browsing history, no cookies, no analytics. Your settings are stored locally in your browser and never leave it. The only other network request is an optional error report you trigger yourself.

The extension is open source, so you can verify this yourself: https://github.com/alike03/SubscriptionInfo (see src/page/lib/api.ts). The declaration didn't change any behaviour, it just makes existing behaviour visible.