WebmailCompose -- FireFox Extension
Over the last couple of days I’ve become quite taken with a FireFox extension called WebmailCompose ( WebmailCompose XPI) This addin for FireFox (although it has an issue installing on 0.9.3) and Mozilla overrides the default behaviour of mailto: links and instead calls your webmail application of choice.
By default support is provided for, among others, Gmail (WOO!), Yahoo Mail, Hotmail and you can even add your own service of choice with a little knowledge of URL formats.