O que eu consegui encontrar foi:
Configurando o cliente de e-mail do Firefox
- Na parte superior da janela do Firefox a barra de menu, clique nas Ferramentas Menu Firefox Edit, e selecione Opções ... Preferências ....
- Selecione o painel Aplicativos.
- Pesquise pelo tipo de conteúdo: mailto e selecione-o.
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Clique na coluna Ações no mailto row, para alterar a ação.
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Ouleiaisto:
Thedefaultmailclientistheprogramthatopenswhenyouclickalinkinanotherapplicationtosendamailmessage,forexample,whenyouclickonamailtolinkorwhenyouclickFile->Sendlink" in Firefox. Note that the default mail client is not as much of an issue when using Mozilla Suite/SeaMonkey because the browser and mail components are integrated (e.g., a mailto link in a SeaMonkey browser window should open SeaMonkey Mail, regardless of your default mail client).
This article describes how to configure Thunderbird as the default e-mail client. However, you must also configure any other e-mail clients you run to not attempt to be the default e-mail client or it will have no effect. For example, in Outlook Express you may need to press a button in "Tools -> Options -> General -> Default Messaging Programs" to make the dialog box say "This application is NOT the default mail handler". Note that the calendar in Outlook will not work unless Outlook is the default email client.
You do not need to make Thunderbird the default email client in order to send/receive email. It's used mainly to allow other applications to use Thunderbird to send a message by making SimpleMAPI calls (if your operating system supports MAPI) , or by your clicking on a mailto: URL (in a browser). It also associates Thunderbird with the .EML file extension under Windows (if you're running 1.5 or later) so that if you double click on a .EML file its displayed in Thunderbird.
Note: If you receive a message such as Firefox doesn’t know how to open this address because the protocol (mailto) isn’t associated with any program. that means that no mail client is currently set as the default and you will need to set one.
In Thunderbird, go to "Tools -> Options -> General" and select "Use Thunderbird as the default mail application". (In the Mozilla Suite, go to "Edit -> Preferences -> Mail & Newsgroups" and select "Use Mozilla Mail as the default mail application".) If that doesn't take effect right away, try restarting Thunderbird or even the computer once.
If that still doesn't work then another email client (such as Outlook or Windows Mail) is probably also configured to be the default email client. If you're using Windows Vista go to "Start -> Default Programs -> Set Program Access and Computer Defaults" and set "Mozilla Thunderbird" as your default e-mail application. See this article at msdn.microsoft.com for more information.
If you're running Windows XP another solution would be to download the DefaultMail utility and use it to set the default e-mail client. It can also do this on a per-user basis. That's useful if several people share a machine (with their own Windows user account) and each wants to use a different e-mail client as the default e-mail client. If none of these methods work it might be because your windows account doesn't have the rights to modify the registry. Try using the "run as" command to temporarily run a command/application as a administrator.