Outlook.com users are shut out of the Firefox party, which is surprising since Mozilla's browser is usually highly customizable. But in this case, your best bet is to use an add-on such as Live Mailer that is specifically built to automatically switch all mailto links in Firefox to Outlook.com.
The latest versions of Opera have the same underlying technology as Chrome. Opera has not built-in mailto handling the way Google has, but the same Mailto: extension you can use in Chrome is also available for Opera and works with all major webmail services.
Microsoft's IE is not as friendly to personalization as other browsers and does not have a mailto setting you can tweak simply and easily. If you're a Gmail user, you can install Google's toolbar for IE that will allow you to set Gmail as your default mail client. Check out previous webmail tutorial for further instruction.
If you're not a Gmail user, you may want to consider switching to a browser like Chrome and Firefox.
Using a webmail service as your default isn't quite as seamless as choosing a desktop program. But with the many add-ons and built-in capabilities of modern browsers it's still a lot easier than configuring IMAP or POP3 in Outlook or Thunderbird.
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