Adding the add-ons
Launch Firefox and go to Tools -> Add-ons and the "Add-ons" windows will appear. You then go to the "Get Add-ons" tab and you can then find any and every add-on that was ever made for Firefox that Mozilla has tested and deemed safe for users to use. There are sites that offer the add-ons directly. However, getting them directly from Mozilla is a safe bet as they have been tested to work correctly "as advertised".
The best way to know what add-ons you want is to simply use the browser and ask yourself what improvements you would like to see to Firefox. Once you know what you want, simply figure out what keyword would best describe the feature you want and query in the "Get Add-ons" tab. The query result will display all add-ons that fit the search query and will then display a short description of what the add-on does. If the description fits what you want, you are then given the opportunity to install the add-on. You can also check the "Extension" tab to see what add-ons you have installed. You can also configure the behaviour of the extension to better fit your needs.
Extension/Add-ons that I use
The list below are the extensions that I use. It is in no way an endorsement to the following add-on authors but it is a list of add-ons that I use that I find useful and convenient. They are:
- Ad Block Plus
- Download Statusbar
- Server Spy
- SwitchProxy Tool
To know what these tools do, may I suggest you query them in the "Get Add-ons" tab and have a read yourself.