Plug-ins were essential to web browser back in the early days of the Internet, because they dramatically expanded the functionality of the browsers by adding support for things like Java and Silverlight. The downside to plug-ins, however, was that they caused numerous performance and stability issues, and were ridiculous susceptible to hacking, which is why they’ve largely been replaced by things like HTML 5. Now that there’s a viable replacement for plug-ins, Google has decided to remove them from Chrome entirely, and Mozilla has announced that it’ll be doing the same thing with Firefox by the end of next year. It’s also important to note that plug-ins are completely separate from add-ons.