The desktop file for thunderbird lacks a tryexec line. This means if the desktop file is copied to a system that lacks thunderbird installed (say in the users ~/.local or ~/.gnome2/panel.d) you'll get a broken icon instead of a nice, dynamically hidden icon.
This request was evaluated by Red Hat Product Management for inclusion in a Red
Hat Enterprise Linux major release. Product Management has requested further
review of this request by Red Hat Engineering, for potential inclusion in a Red
Hat Enterprise Linux Major release. This request is not yet committed for
Should it replace the Exec line or should it be additional?
Additional. It's going to be included in 3.0.5 security update.
TryExec is present
Red Hat Enterprise Linux 6.0 is now available and should resolve
the problem described in this bug report. This report is therefore being closed
with a resolution of CURRENTRELEASE. You may reopen this bug report if the
solution does not work for you.