Icedove is a PLD Linux effort to create freely distributable version of Thunderbird e-mail client. Icedove is almost completely compatible with Thunderbird. Most add-ons designed for original application will also work with this one.
NOTE: PLD Linux Icedove differs from Debian project with the same name.
There are many useful extensions on mozdev.org. Make sure to check whether the add-on is compatible with Thunderbird and is not specific for Windows platform. You should be able to install most of them in your personal directory.