The Web-Mail extension creates a platform which other extensions use to integrate web based email accounts into Mozilla Thunderbird. POP is the only protocol supported, this means Thunderbird can only download emails.
Update 2005-04-11: Thanks to hints below I've found the option to add webmail accounts in File > New > Account... Any documentation (general or otherwise) on what providers can be accessed by default?
Update 2005-10-04: This extension broke with the Thunderbird 1.0.7 update. (Re)installing 1.0.6 should bring everything back to good. Be advised that older versions of the client lack security updates found in 1.0.7.