What are Add-ons/Extensions and How to install Add-ons/extensions Offline in Firefox and Google Chrome?

About:

Everyone using these heavy Browsers (Firefox and Chrome) Should know about their add-ons/extensions.
Add-ons are small pieces of software that add new features or functionality to your installation of Firefox/Chrome. Add-ons can augment Firefox/Chrome with new features, foreign language dictionaries, or change its visual appearance. Through add-ons, you can customize Firefox/Chrome to meet your needs and tastes.

Official websites for installing Add-ons and other types of Apps:

You can Install Add-ons in Firefox from https://addons.mozilla.org and for Google Chrome You can install extensions from https://chrome.google.com/webstore

How to install extensions Offline In Firefox And Google Chrome:

 You have to download that Add-on/extension manually (You must have IDM or other Download manager) to save it in your computer. For this...

• Go to Download/install page of Addon.
• Right-click on Add to Firefox/Chrome and then Click on Download with IDM. [see screen shot of this step]
• Now, IDM will download that add-on/extension.
• Open folder in which you have downloaded the file.
• Now, Open Firefox/Chrome.
• If you are using Firefox Press Ctrl+shift+A and if you a chrome user then Click on upper-right button>Tools>Extensions to view the extensions/add-ons window.
• Now, Drag and drop that Extension in the Opened Window of Firefox/Chrome. [see screen shot of this step for firefox] [see screen shot for Chrome]
• Click install and enjoy the fun!