Question:

Firefox does not support the mp4 video file format. How do I add support for the webm video file format?

Answer:

You can use data tag data-webm to specify a webm video for Firefox. For example:

<a href="images/Big_Buck_Bunny_2.mp4" data-webm="images/Big_Buck_Bunny_2.webm" class="html5lightbox" >Video</a>

If the data-webm is not specified, the Lightbox plugin will use Flash to play mp4 on Firefox, so it will still work.