In a few steps I will show you how to install additional extensions (add-ons, plug-ins) on you Thunderbird e-mail client.
Maybe you are already are using some Mozilla Firefox add-ons, and you want to install some useful at your Mozilla Thudnerbird.

There is a little bit difference between these two applications, as the browser (Firefox) installs them directly unlikely from the mail client (Thunderbird) where you need to install them manually.

If you do not have a noticed extension, you can download many from the official Mozilla website: https://addons.mozilla.org/en-US/thunderbird/

There choose an add-on and save it to your local hard drive.
For this tutorial I chose “Mail redirect” used in Restore already downloaded POP mail messages. I have to mention that this add-on is working only on Thunderbird 2.0!

Anyway, just choose an extension and save it on your PC. Then open Thunderbird and go to:

Tools > Add-ons

This will open the Thunderbird Add-ons section. There click on install and locate the downloaded extension on your hard disk and choose it and press OK.

A new pane will appear informing you what will be installed.

Wait the ‘Install’ button to be available and click on it. There is one sing saying: “ You should only install software from sources that you trust.” - and you really must keep that in mind.

Installing malicious software may compromise your personal data even that the Mozilla Thinderbird is one of the securest mail clients.

Anyway, when you accept the installation the new extension will appear in the list of the add-ons.

Then just click “Restart Thunderbird” in order the new changes to take effect.

When the client is restarted, the Thunderbird add-on will appear in it according to what is it doing.
In this particular case a new option will appear in ‘Messages’ section.

Another thing that must be mentioned is that you have several supplements available in the Add-ons section in Thunderbird.

According to the extension type there you may have ‘Options’ from where to make adjustments with the plug-in and there are always ‘Diable’ and ‘Uninstall’ buttons giving you ability to stop or just get rid of the installed add-on.

As you can see from the image above, you have ‘Get Extensions’ link which opens the official Thunderbird extensions website as well as ‘Find Updates’ button which helps you to keep the installed software up to date.