http://support.mozilla.com/en-US/kb/Changing+the+e-mail+program+used+by+Firefox

Outlook is already set up as my default program for e-mail, both for Firefox and my System. All other browsers can do the mailto link out trick with no problem. My recent update may be corrupted/damaged or some setting is wrong.

Did you check the Mailto settings in Tools > Options > Applications ?

You can look at the network.protocol-handler.external.mailto prefs on the about:config page.
If network.protocol-handler.external.mailto prefs are user set (bold) then right-click and Reset them to the default value.

To open the about:config page, type about:config in the location (address) bar and press the "Enter" key, just like you type the url of a website to open a website.
If you see a warning then you can confirm that you want to access that page.

"Did you check the Mailto settings in Tools > Options > Applications ?

You can look at the network.protocol-handler.external.mailto prefs on the about:config page. If network.protocol-handler.external.mailto prefs are user set (bold) then right-click and Reset them to the default value.

To open the about:config page, type about:config in the location (address) bar and press the "Enter" key, just like you type the url of a website to open a website. If you see a warning then you can confirm that you want to access that page.

'This worked great! This issue was pissing me off, I was familiar with about:config page but this shit had me stumped! Thanks cor-el '

Yes, I'm having this problem, too. But I'm on a Mac, running Lion and Firefox 11. My mailto client is set to Outlook (in Office 2011 for Mac) yet Firefox just opens a blank tab. In Firefox 10.x, it tried to open my gmail account. What's up - all my settings are correct, yet my mailto client is simply ignored: opening the blank tab.

You know, my problem is that the Mailto icon in Options>Applications is that funky unrecognized icon. it's not that it's not there, or that it's not set, it's totally unrecognized. A bunch of other applications are also unrecognized, like irc, ircs, magnet and webcal. Also, most of the files in my profile folder are unrecognizable too. What's the story? What I gotta do?

network.protocol-handler.external.mailto

Hey - that worked! I've been asking this question for months and now finally found an answer. Thanks so very much! It works on a Mac running Lion, Firefox 11, and Outlook, at least. No more trying to load GMAIL or opening blank tabs. I owe you a latte, my friend.

I've tried everything - fixed the 'network.protocol-handler.external.mailto' thingie and still didn't work. BTW, in my 'Tools > Options > Applications' I don't have Gmail - just Googlemail - I suppose it's the same thing? Anyway Googlemail doesn't work. Tried 'Use handlemailto' which uses "GmailDefaultMaker.exe" and it worked!!

What OS are you running? Are you running Firefox 12? The "about:config" - "network.protocol-handler.external.mailto" was set to bold (false) and I just double-clicked it, setting it to "true" and my mailto opens Outlook all the time now.

I'm running W7 and Waterfox 11.0 (the 64bit version of Firefox). When I was using FFox 11.0 I had "network.protocol-handler.external.mailto" true, didn't work - changed to false, still didn't work. Same in Waterfox. It seems the 'Use handlemailto' which uses "GmailDefaultMaker.exe" was the only way I could fix it and this was a 'now I'll try anything' last ditch lucky guess that worked. Pure luck and guesswork but it works now. I've tried everything I've ever seen on these forums (fora?) and nothing else worked for me.

I found a solution. It's sort of a "hold your nose the right way" solution but it worked for me when everything else failed. I went into preferences => applications and found the mailto entry. It was set to "Thunderbird (default)" but that wasn't working. I then set it to always ask. The next time I hit a mailto link it asked and I told it to use Thunderbird and remember my choice. After that FF worked the way I expected.