<system.web>
       <customErrors mode="Off"/>
   </system.web>

   <system.web>
       <customErrors mode="RemoteOnly" defaultRedirect="mycustompage.htm"/>
   </system.web>

I have been trying to make Outlook.com my default mailto. So far I've followed the advice to download this add-on https://addons.mozilla.org/en-US/firefox/addon/livemailer/ It does not work for me even after restarting Firefox. Then followed jscher2000's post of 3/20/2013 5:39 pm to edit the about:config , then GECKO from this link https://support.mozilla.org/en-US/questions/953800 The first time I did this I got an error page. Then realized that from here in Canada we have to go to blu175.mail.live.com for our hotmail email, so the 2nd time I attempted this I edited the mail.live.com part of the string to paste to include blu175. This also fails to work and I get the error page again. Here is what appears in the URL when I click on the link to do a mailto email. https://mail.live.com/secure/start?action=compose&to=mailto%3Ainternetm%40vrtp.com.au%3Fbody%3DDear%2520Village%2520Roadshow%252C%2520%250A%250AI%2520am%2520incredibly%2520concerned%2520to%2520learn%2520Village%2520Roadshow%2520Theme%2520Parks%2520and%2520its%2520subsidiary%2520Sea%2520World%2520Australia%2520have%2520partnered%2520up%2520with%2520a%2520Chinese%2520company%2520to%2520manage%2520the%2520new%2520Hainan%2520Ocean%2520Paradise%2520marine%2520park%2520in%2520China.%250A%250AA%2520Sea%2520World%2520Australia%2520representative%2520recently%2520stated%2520they%2520%25E2%2580%259Ccould%2520not%2520rule%2520out%25E2%2580%259D%2520taking%2520animals%2520from%2520the%2520wild%2520to%2520stock%2520this%2520new%2520marine%2520park%2520%25E2%2580%2593%2520including%2520beluga%2520whales%2520captured%2520in%2520violent%2520Russian%2520hunts.%250A%250AIf%2520you%2520go%2520ahead%2520with%2520buying%2520wild%2520dolphins%2520and%2520whales%2520for%2520captivity%252C%2520it%2520will%2520drive%2520up%2520demand%2520and%2520keep%2520these%2520horrific%2520hunts%2520in%2520business.%2520You%2520would%2520be%2520signing%2520a%2520death%2520warrant%2520for%2520countless%2520innocent%2520dolphins%2520and%2520whales.%250A%2520%250AOn%2520its%2520website%2520Sea%2520World%2520Australia%2520says%2520it%2520cares%2520about%2520animals%252C%2520and%2520does%2520not%2520take%2520dolphins%2520from%2520the%2520wild.%2520It%2520also%2520states%2520%25E2%2580%259CAny%2520international%2520marine%2520park%2520we%2520operate%2520will%2520uphold%2520these%2520same%2520values%2520and%2520ethics%2520as%2520Sea%2520World%2520Australia%25E2%2580%259D.%2520%250A%250APlease%252C%2520don%27t%2520support%2520the%2520global%2520animal%2520cruelty%2520trade.%2520Come%2520clean%2520about%2520where%2520the%2520animals%2520for%2520your%2520new%2520marine%2520park%2520will%2520come%2520from%2520and%2520promise%2520that%2520no%2520marine%2520park%2520under%2520the%2520management%2520of%2520Village%2520Roadshow%2520or%2520Sea%2520World%2520will%2520take%2520animals%2520captured%2520from%2520the%2520wild.%250A%2520%250ABecause%2520animal%2520abuse%2520is%2520wrong%2520in%2520ANY%2520country.%26subject%3DVillage%2520Roadshow%2520and%2520Sea%2520World%2520Australia%2520%25E2%2580%2593%2520don%27t%2520support%2520global%2520animal%2520abuse Here is the text of the messages that appear on the screen at the above link Server Error in '/' Application. Runtime Error Description: An application error occurred on the server. The current custom error settings for this application prevent the details of the application error from being viewed remotely (for security reasons). It could, however, be viewed by browsers running on the local server machine. Details: To enable the details of this specific error message to be viewable on remote machines, please create a <customErrors> tag within a "web.config" configuration file located in the root directory of the current web application. This <customErrors> tag should then have its "mode" attribute set to "Off". <!-- Web.Config Configuration File --> <configuration> <system.web> <customErrors mode="Off"/> </system.web> </configuration> Notes: The current error page you are seeing can be replaced by a custom error page by modifying the "defaultRedirect" attribute of the application's <customErrors> configuration tag to point to a custom error page URL. <!-- Web.Config Configuration File --> <configuration> <system.web> <customErrors mode="RemoteOnly" defaultRedirect="mycustompage.htm"/> </system.web> </configuration> OK I'll admit I do not understand any of that, except that something went really wrong. Both of the live.com choices appear when I load the page to choose the default mailto program so at least adding them as choices worked for me. I'm left wondering if there is a problem with either the link in the email to do the mailto, or have I really messed up Firefox? The email is not private so I would be willing to forward a copy to anyone willing to help me sort this out and get Firefox to allow me to use outlook.com for all mailto links.