In Mint the MPlayer plugin for firefox is installed by default. So you must remove it for using the VLC plugin. If not it's the MPlayer plugin that Firefox will use.
Also with a right click in a tab using the MPlayer plugin, you can, I guess, select the media type that you want use with the MPlayer plugin.
K.I.S.S. ===> "Keep It Simple, Stupid"
"Simplicity is the ultimate sophistication." (Leonardo da Vinci)
"Everything should be made as simple as possible, but no simpler." (Albert Einstein)