First install Thunderbird:
sudo aptitude install mozilla-thunderbird
Act upon the info given here.
So install the webmail plugin by going here and downloading the xpi file to a local folder (right click and save as). You first have to install the Webmail add on and after that the GMail add on (or Yahoo etc.):
- In Mozilla Thunderbird, open the extension manager (Tools Menu/Extensions).
- Click the Install button, and locate/select the file you downloaded and click "OK".