Here is the Addon's Link -> https://addons.mozilla.org/en-US/firefox/addon/9gag-media-download-helper

Instruction on how to use:

• To get the most out of this add-on, open a post/gag in a separate tab then download the image/video by pressing right click on the image/video and selecting the "Download 9gag" option.
• A download pop up should open prompting you to select the location. The file name will be identical to the title of the post.
• Both images and videos can be downloaded from from main page as well. But the title as file of the videos will work only when the video/gif is opened in a separate tab.