I have used Video DownloadHelper 4.6.4 firefox plugin for this post. But any firefox plugin can be installed with similar steps.

PART I – Installing (2 Methods)

METHOD (1) – “Add to Firefox” – STEPS for installing the plugin directly on firefox:

Step 1. Open Firefox Add-ons website by following this link: Video DownloadHelper 4.6.4. Make sure this website is open with firefox browser.

Video DownloadHelper 4.6.4 Firefox Plugin

Step 2. On the plugin web page you will see a green button named “Add to Firefox”. Click on that button.

Add to firefox button

Step 3. The “Software Installation” window appears. Click “Install Now” button.

Firefox Plugin Software Installation Window

Step 4. The video download helper starts to download. Wait till the download finishes in few seconds.

Video DownloadHelper downloading

METHOD (2) – “Download the .xpi file” – STEPS for downloading the plugin as a .xpi file and installing:
Step 1 . Open Firefox Add-ons website by following this link: Video DownloadHelper 4.6.4. Make sure this website is open with any other browser than firefox.

Video DownloadHelper 4.6.4 Firefox Plugin on non firefox browsers

Step 2. On the plugin web page you will see a green button named “Download Now”. Click on that button.

Download Now Firefox Plugin Button

Step 3. Save the “downloadhelper-4.6.4-fx.xpi” file somewhere on your computer. You will need this file to do the installation later.

Step 4. Open a blank Firefox browser window.

Step 5. Drag to Firefox window, the “downloadhelper-4.6.4-fx.xpi” file that you had downloaded in step 3

Step 6. The “Software Installation” window appears. Click “Install Now” button.

Firefox Plugin Software Offline xpi Installation Window

PART II – Restarting and finalizing installation

Step 1. Click “Restart Firefox” button to finish the installation and restart firefox.

Restart Firefox after plugin installation

Step 2. If the following window shows up click “Restart” button.

Restart Firefox restore tabs

Step 3. Firefox closes itself. Wait till firefox restarts on its own. When Firefox loads again, the plugin should be installed and ready to use (If the Add-ons window appears, just close it) .