I wrote a few days ago 7 reason why you should use firefox (7 beeing a magic number).
Here is what you will learn from this article:
1.What is and how to install plugins / add ons
2. How to configure / update / delete add ons
3. 3 Must Have Plugins for Fun, Utility and Safety
1. What is a Firefox plugin / add on?
First of all a plugin or an add on is the same thing. You can see the plugins installed from Tools> Add-ons.
So basically an add on is a little piece of software that runs from and into your firefox browser. Using add-ons you can have in one small application lots of other applications that will run from a single click.
The advantage is that you don’t have to scroll, search and open other programs from other places of your computers. You can have all you need in your browser.
A disadvantage is that you have lots and I mean lots of plugins then this may slow down your browsing - so be carefully and install just what you need.
2. How to configure / update / delete add ons
Go to: Tools > Add ons. Select the plugin you are interested. Look at the picture:

Now select the button you want (delete, options, disable). If you want to update press the update button (bottom right).
3 Must Have Plugins for Fun, Utility and Safety
Here are 3 add ons that I recommend you to use.
For Fun and Domains of Interes go to http://www.stumbleupon.com/ and make an account. There you will find a link to an Firefox Add-On if you don’t find it go here.
Click install now. If a yellow bar will appear on top click it and hit Allow. After this click again install now. This window should appear.

Wait until the button install becomes active and then press it. After installation restart Firefox and a toolbar with stumbleupon will appear. Set your preferences (domains of interest) log in and start using it by pressing Stumble!. If you like a site press I like it! if you don’t press thumb down.
For hiding or showing the toolbar press Ctrl+F11.
Fore Utility .
If you have one gmail account or more this is the plugin for you - Gmail Manager. With these plugin you will be able to configure, access and use all your Gmail accounts.
Allows you to manage multiple Gmail accounts and receive new mail notifications. Displays your account details including unread messages, saved drafts, spam messages, labels with new mail, space used, and new mail snippets.
Here is the link for this nifty plugin. Use the same steps like you did bellow. After this restart Firefox and go to preferences and click Add to add you accounts. The plugin will be visible in your Firefox status bar.
For Safety
Fore safety I recommend you to install adblock and block all annoying pop-ups and banners.
Right-click on a banner and choose “Adblock” from the context menu - the banner won’t be downloaded again. Maybe even replace parts of the banner address with star symbols to block similar banners as well. Or you select a filter subscription when Adblock Plus starts up the first time, then even this simple task will usually be unnecessary: the filter subscription will block most advertisements fully automatically.
This plugin will also be visible in the right corner. Right click on it and select list all blockable elements. Look at url-s that are starting with ads, ad server and so on. Select them an click ok. If you done something wrong go to preferences and delete them from there. Here is how my adblock looks.
You can put the same filters. This are one of the biggest networks out there that can be found on many sites.

Anyway if you want sometimes to see ads, right click and select disable. Be careful what you block.
Also you may find easier to use this plugin (adblock plus). Test them and decide what you will use. My recommendations goes for adblock because you have the most control and you will not let the software to decide based on one image.
Now you know how to install, update, disable, delete and configure Firefox add ons (or plugins). Be sure to practice and browse safety!
More to come. For any questions please comment below!