FireFox is not rendering correctly audio and video files if they are not streamed from the server with the correct mime-type.
Often some servers streams the ogg and webm files with the mime-type application/octet-stream. To solve this issue on linux webservers you just need to upload in the root directory of the server an .htaccess file containing following instructions:

AddType audio/ogg oga ogg
AddType video/ogg ogv
AddType video/webm webm