Is GMX MailCheck for Firefox safe?

GMX MailCheck for Firefox ships hardcoded OAuth2 client secrets and may silently enable telemetry during version upgrades.

The extension bundles Base64-encoded OAuth2 client credentials for four mail providers (GMX, GMX.com, mail.com, WEB.DE) directly in its publicly inspectable source. When upgrading from versions older than 8.0.0, it force-enables full telemetry collection without prompting the user, and subsequently sends persistent identifiers, account counts, locale, and default-search status to tgw.gmx.net on a daily, weekly, and monthly schedule.

Part of this rating comes from analysis signals we haven't published as detailed findings yet.