If you're using Firefox on your Nexus 9 (or any other Android 5.0 device), you might experience issues playing video and audio on some sites. Most of these issues have been resolved on the latest version of Firefox. Just follow these simple steps.
I can't play videos on Firefox for Android
Update to Firefox version 34 (currently on Beta). Download the Beta version on Google Play. If you already have Firefox on Beta, follow these steps to update to the latest version. This issue has been resolved on your version of Firefox. If you're still experiencing problems, please contact support.
I can't play MP3 files directly on the Firefox for Android browser
This is a known issue that Mozilla is trying to resolve. In the meantime, try downloading the MP3 file and play it in your native MP3 player instead.