Firefox on Android (v60) doesn't have any interface to import client certificates. I got it working by manually importing the certificate into the NSS database by doing this:

$ adb root
$ mkdir p12_import && cd p12_import
$ adb pull /data/data/org.mozilla.firefox/files/mozilla/%%%/key4.db
$ adb pull /data/data/org.mozilla.firefox/files/mozilla/%%%/cert9.db
$ pk12util -i /path/to/cert.p12 -d sql:/path/to/p12_import/
$ adb push key4.db /data/data/org.mozilla.firefox/files/mozilla/%%%/
$ adb push cert9.db /data/data/org.mozilla.firefox/files/mozilla/%%%/
$ adb shell
> cd /data/data/org.mozilla.firefox/files/mozilla/%%%/
> chown $$$:$$$ cert9.db
> chown $$$:$$$ key4.db
> chmod 600 cert9.db
> chmod 600 key4.db
> exit
$ adb reboot

You can also tell Firefox to automatically select the client cert with this setting in about:config:

security.default_personal_cert: Select Automatically