Plugins in browser is like a double-edged sword. In other side, it can enhance your browser's capabilities, but in the other hand, it could be a gateway to attack your system. The only solution is to use the latest version and stay up to date.
Mozilla Firefox has been quite nice to have a Plugin Check service which allows us to check our plugins. All you need is to visit the site and it will list all the plugins you have on your system. Another service that you can use and it's very easy to use is Browser Check from Qualys. Both are working in Linux platform as well (and probably Mac OS X too, but i don't have Mac-based machine, so i will leave it to anyone who can confirm this).