Firefox makes the distinction of being one of the few browsers to make Germans happy after the Bundesamt fur Sicherheit in der Informationstechnik tested Mozilla Firefox 68 (ESR) against Google Chrome 76, Microsoft Internet Explorer 11, and Microsoft Edge 44.
They did not bother to test Safari or Opera. It is not clear why because Opera has pretty good security standards. The audit was carried out using the very precise rules detailed in a guideline for "modern secure browsers" that the Bundesamt fur Sicherheit in der Informationstechnik published last month.
The BSI normally uses this guide to advise government agencies and companies from the private sector on what browsers are safe to use.
The article includes a list of all the minimum requirements required for the BSI to consider n browser "secure". It also lists the areas where the other browsers failed, such as lack of support for a master password mechanism (Chrome, IE, Edge), no built-in update mechanism (IE), and No option to block telemetry collection (Chrome, IE, Edge).