I have some .crt files exported from my desktop firefox. When I opened one from the firefox for android, it never asked me to import it to firefox. Instead it just downloaded the file and when I opened it, it was installed to android device but not firefox. I checked out this https://mtersch.wordpress.com/2015/03/17/certificate-import-in-firefox-on-android/ and used the same python web server code and it didn't help. When I opened the the crt file from file:///sdcard, it just opened it as a text file.