I have seen this in the past. The Webmail 1.4.1 is setup for Thunderbird and not for Firefox. You need to save the file and manually install in TB5.0. Note this version will not with older versions of TB. If your using an older version of TB and you already have Webmail installed then it should be update automatically during the upgrade to TB 5.0.
You will need Webmail 1.3.13 for the older versions ( at least back to version 2.0). Eugene
