https://antportal.com/wiki/installing_client_certificates/

Scroll all the way down to Android. This should answer your question.

That is incorrect information. Firefox manages certificates itself. Firefox does not use the Android certificate databases.

Firefox should prompt to install the file if you can access it from the web. I used the files from https://bugzilla.mozilla.org/show_bug.cgi?id=550041 to test.

Firefox import Certifcate Authority not user. You need to create AddOn. see my reply (in french forum) French Mozilla Forum

// Sample Function

{

   var wm = Cc["@mozilla.org/appshell/window-mediator;1"].getService(Ci.nsIWindowMediator);
   var browserWindow = wm.getMostRecentWindow('navigator:browser');
   var certDB = Cc["@mozilla.org/security/x509certdb;1"].getService(Ci.nsIX509CertDB);
   var filePicker = Cc[ "@mozilla.org/filepicker;1" ].createInstance(Ci.nsIFilePicker);
   filePicker.init( browserWindow, "Import Certificate", Ci.nsIFilePicker.modeOpen );
   filePicker.appendFilter( "Certificat","*.p12");
   var result = filePicker.open(function(e){
   if (e==Ci.nsIFilePicker.returnOK)
       {
       var theFile = filePicker.file;
       certDB.importPKCS12File(null, theFile);
       preferences.setBoolPref("installed", true);// For next launch...
       }
   });
 }

Warning , a bug block certificate usage. see : [https://bugzilla.mozilla.org/show_bug.cgi?id=921477]

• status-firefox26: --- → affected
• status-firefox24: --- → affected
• status-firefox27: --- → fixed
• status-firefox25: --- → affected

It's work with Firefox 23 or the Nightly Version.