unable to install webmail

When I try to install webmail by downloading the
web-mail-1-0-9.xpi 
extension in Firefox, I get the following error:

   WebMail 1.0.9 could not be installed because it is not
compatible 
with Firefox.

I am using Firefox and Thunderbird version 1.5.0.4 (i.e.,
most recent 
version) -- should the WebMail extension be compatible with
these most 
recent versions?

Many thanks,

Roger Levy

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