Thanks Cheah Chu Yeow! I found this useful Firefox extension (GMailCompose), which adds a "Compose GMail" option to the context menu for mailto: links and text addresses, from his post about Firefox BBCode extension.
Thanks man! Believe it or not, my friend Kwan actually asked me whether this extension exists on this Tuesday (i.e. 2 days ago).
Of course thanks to Jed Brown who wrote this little extension at the very same Tuesday (version 0.2.2 was released at 29 June 2004).