The settings are shown in the screenshots below. Picture speaks a thousand words. I have blanked out my information to protect the innocent (me). Click on the screenshots if they are too small, and it will show up bigger.
I did not use the Webmail add-on for Thunderbird (in fact, I tried it and it was not working too well to the point that I uninstalled Thunderbird and start from scratch again - giving it a last try before abandoning Thunderbird altogether).