thanks for that Greeknight.In the default bookmarks that come with Firefox, in the "Firefox and Mozilla Links" folder, there's one called "Extensions and Themes". This takes you to the official Mozilla Addons site, which is the safest place to get them.
It's best to get the hang of Firefox without adding extensions or themes first. Search the forums on Mozillazine (there's a link to it in the same bookmark folder) for information on extensions you might be interested in. Install extensions or themes one at a time, and use each one for a while to make sure there are no problems before installing another.
Also, avoid installing different extensions with overlapping functions; if you have two extensions that are both trying to modify the same function, it can create a conflict that causes big problems.