FirePHP updates the User-Agent request header by appending "FirePHP/
X.X.X" to the end of it if enabled. Take a look at the
FirePHP.class.pgp file in the FirePHPCore library which you can
download from
http://www.firephp.org/HQ
By default the FirePHPCore library does not send any headers if
FirePHP is disabled.
Christoph