To remove an unwanted add-on follow the steps below:
1.Click On Tools
2.Click On Add-Ons
3.Click On The Extensions Button
4.Click on Add-On you wish to uninstall
5.Click On Uninstall

To install a Firefox Add-On follow the below steps:

1. Go to the website containing the Firefox Add-On, the Mozilla Add-On website contains many add-ons available for free.

2. Click on the button which says Add To Firefox (Mozilla Add-On website) other websites may have different text on the button.

3. (Skip to Step 4 if you are downloading the add-on from the Mozilla Website) If you are downloading from a personal website website you will be asked to verify that website.  Make sure a website is a trusted one before installing.

4. Once you click the button a window will appear with a countdown on the install button wait for the countdown and then click install.

Click here to view My Top 5 Firefox Add-Ons