The webmail 1.4.1 extension is for Thunderbird 5.0 and will not install in Firefox. You will need to save the file and manually install in TB5.0 If your using an older version of TB then you will a different version of Webmail in order for it to work.
This has been an ongoing problem as Firefox which also uses xpi extensions is currently unable to tell the difference until it tries to install the file. Eugene
