How to Install Add-on Extension:

The ideal case

In an ideal world, all the add-ons you would want are officially vetted, approved, and can be found on the official website, Addons.mozilla.org .

For those in this category,You simply go to the Add-ons manager tab in Thunderbird, using the toolbar entry then and enter the add-on's name into the search box.

Here is a search for Add-ons that mention contats:

Once you locate an add-on you like, click on Install, wait for it to be downloaded, then click on Restart Thunderbird if prompted.

A slightly less ideal case

You may have been given an URL for the add-on, or the add-on you want isn't on the "official" page. Searching for it, as shown above, doesn't locate the your addo-on.

Visit the webpage in your browser, and left click the download link with your mouse and select "save link as". and save the XPI file to a location on your computer that your can find it again. As the download is temporary it is suggested you use either your downloads folder or your desktop.

To install a downloaded XPI file into Thunderbird you really have two options.

Download

Right-click the link and select Save As or Save Link As as discussed earlier.

You will be invited to download and save the file. Most browsers have a default download location, often your Desktop. Make a note of where it downloads to, and what it's called. Some add-on files have silly names like “latest-234.xpi” which is ponderously unhelpful when you're trying to find it later on.

Open the Add-ons window in Thunderbird. There's a button to the left of the search window. Click this and from the following menu select Install Add-on From File.

You will need to navigate to where you saved the XPI file in the file picker and select it to complete the install.

Drag-and-drop

• Open the add-ons manager ( then )
• Drag your downloaded XPI file to the add-on manager tab and drop it.