I like to keep things simple. Here's some simple but useful Add-ons for Thunderbird. Just go to Tools -> Add-ons -> Get Add-ons and search for these:
Google Calendar Tab - Adds a button to your Thunderbird Toolbar. Click it and it opens a new tab displaying your Google Calendar in normal browser view. simple.
Toolbar Buttons - An assortment of new buttons that you can drag and drop onto your Thunderbird Toolbar to customize it to your liking. Add a Send/Receive button just like Outlook. I also find the "Toggle Read" button useful to quickly mark emails as "unread" again.
And of course, Google Contacts to keep your Thunderbird contacts synchronized with GMail and your Android phone! The Best Way to Keep Outlook Express Address Book Automatically Synchronized with Google Contacts
Are you tired of your Add-ons getting disabled every time Thunderbird gets a new update? In most cases, this occurs not because it's no longer compatible with the new version of Thunderbird, but simply because a property file for the plugin is reporting an incorrect version range. Here's the best Add-on of all: Add-on Compatibility Reporter - Keeps your Add-ons active after a Thunderbird update. You can then use them and decide to disable them only if they truly no longer work.
Maybe I should have called this blog Thundernaut instead? ;-)