The mozilla-firefox-1.5-r9 ebuild has the following build control: mozconfig_annotate '' --disable-mailnews but the Tools menu features the following two items which perform no action: Read Mail New Message... Ctrl+M This is probably an upstream bug but it looks a bit odd for a browser to have these options when Mail and News are disabled.
Firefox 1.0.7 has a mail button for the toolbar with similar options - it open up Thunderbird for me (which is my default mail program, so I can't tell whether it only applies to Thunderbird or the system default mail program). My guess is the menu items perform the same functionality.
They can actually be used to control your mail-client of choice.