The current version of Mozilla doesn't open an external e-mail program such as PocoMail when a mailto link is clicked, using Mozilla Mail as the default even if it is not the default mail application.
There's no way to fix this by using Edit, Preferences to configure the browser. However, Mozilla supports some additonal user preferences that can be configured by creating a short JavaScript file in your Mozilla profiles folder.
ResExcellence offers a tip for changing the network.protocol-handler.external.mailto preference.
Though ResExcellence's tip is for Mac users, it also works on Windows. Find your Mozilla profiles folder (on Windows XP, mine was in a subfolder of \Documents and Settings\Rogers\Application Data\), put the user.js file there, then restart the browser.
Mike Zornek offers a similar solution.
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