Roger,
The plugins are free, and do not take up hardly any space. Because so many programs use different types of languages, plugins will need to be added from time to time when cutting edge technology is used.
To add plugins, the website you are viewing will tell you if it needs one. Click on the link it supplies and it will auto download right into Firefox. They try to make this as easy as they can.
The Adobe Flash plugin is the only one Whitetail Exchange uses. (You need it for Safari as well but usually if your mac was setup by a dealer, he would have added this for you).
Hope this helps.