I've used Firefox exclusively for years and very seldom have to switch over to IE. One example is Windows updates which require IE and another is Netflix downloadable movies.
As for your video plugin, you should see an icon (usually green) notifying you that you need a particular plugin. Simply click on the icon and in most cases the plugin with be downloaded and installed automatically. If not, it will ask you to install it manually. At which time you may be directed to the plugins web site to download it yourself.
