How to install P12 certificates in mobile Firefox?
At this time, Firefox for Android does not support importing a PKCS #12 certificate.  This is bug 868370.

If you have root access to your device, you can import it manually using a Linux computer attached to the phone with adb.

If your client certificate came from a &lt;keygen&gt; form, you can use the same form in Firefox for Android to generate and install a new certificate directly (skipping the PKCS #12 step).