How to save firefox addons for offline installation

On the Firefox addon page search for your desired addon.

Download Addon

  1. Continue to download
  2. Right-Click -> Save Link As... -> save the .xpi file

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Installing Addon

Drop the addon in the adress bar.

Possible. If you need only the Add-ons to be saved & install offline, you may refer below steps. Or if you're planning to do a full backup, you can also refer Journeyman Geek's Answer.

  1. These files are usually .xpi or .jar files.
  2. Download the file to your local computer.

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  1. At the top of the Firefox window, click on the Firefox button, and then click Add-ons. The Add-ons Manager tab will open.
  2. In the Add-ons Manager tab, select the Extensions panel.
  3. To add the downloaded add-on to the list of available add-ons, drag and drop the file into the Add-ons window. The Add-on is added to the list.
  4. The installation process should begin.

That should get your work done.

Source: Mozilla Support