#Firefox #Android (incl Firefox #Klar ) #userCertificates how to:
1) Open about:config by entering chrome://geckoview/content/config.xhtml
2) set security.enterprise_roots.enabled to true

Now if you have installed the user cert itself and its issuing CA as trusted to Android, Firefox will query you to use the user certificate.

WTF... Did I mention I don't trust #Mozilla in any way?