The best thing is to wait for Webmail Author to give you an updated file. However, if you want to try and troubleshoot yourself, do the following:
rename an xpi file (e.g. webmail_1.4.1.xpi) to xpi.zip (e.g. webmail_1.4.1.xpi.zip). Then extract the archive into a folder (the xpi file is actually a zip archive). You will see that in the folder there's a file called "install.rdf". Right-click on it, select "Open with" and choose Notepad. There you go ! You can now modify the xpi's rdf file for whatever reason. You will need to remake the xpi archive after this, by the way. To do this, use WinRar or WinZip and create a "ZIP" archive out of the components of the extracted folder (so don't add the one main folder which you initially extracted to the archive, you must go one level into that folder and create the archive out of its components, such as install.rdf, chrome.manifest etc.). Don't forget to rename the newly created ZIP archive with the "xpi" extension. Good luck!
