To disable a Firefox Add-On follow the instructions below.
1. Click On Tools.
2. Click On Add-Ons.
3. Select Add-On you wish to disable.
4. Click on the Disable button.

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