you can always make a link open up in a new window by pressing the shift-key while clicking on it

I am running a compiled program which takes over the keyboard. While program is running I cannot use the keyboard without causing problems . At completion the program submits the program to the OUTBOX for later submittal.

Why did the mailto function change in version 12? Is it a bug or a Firefox program change?

Also does not work in version 13 beta 1, 2, 3 or 4

Still not fixed in version 13 beta 5 released 5/24/2012

Is the link in the page displayed in the current tab, or are you submitting a URL to Windows, externally to Firefox, and Firefox is failing to create a new tab?

If the link is in the current tab, Firefox by default will open the link in the current tab unless there is a target attribute for the link, or it triggers the JavaScript window.open() method. If the link has target="_blank", which usually open a new window, Firefox by default will divert the link to a new tab. By default, Firefox also will use a new tab for a window opened by a script, unless it has specified size or other feature settings, in which case it will open in a new window. If the link is external, then Firefox generally follows the same setting as a link in a tab.

You can tweak these settings to some extent. I assume you are familiar with the about:config preference editor. If you type or paste browser.link in the filter box, you should see the three settings relevant to the above. The interplay between these seems a little complicated, so apologies for any errors in description.

browser.link.open_newwindow

3 = divert new window to a new tab (default) (checked*)
2 = allow link to open a new window (unchecked*)
1 = force new window into same tab

* First checkbox in Options > Tabs

browser.link.open_newwindow.restriction

0 = open_newwindow preference honored for all new windows
2 = open_newwindow preference honored for all new windows except script windows with features (default)
1 = open_newwindow preference ignored completely (same effect as open_newwindow=2)

browser.link.open_newwindow.override.external

-1 = open_newwindow applies to external links (default)
3 = open external links in a new tab
2 = open external links in a new window
1 = open external links in active tab

You may notice something missing, and that is to open every single link in a new tab regardless of whether the page is designed to launch it in a new window. If you want that feature, you probably need an add-on. If you have been using such an add-on and it stopped working in Firefox 12, either Firefox might have disabled it or there might be a problem with the add-on.

Is any of that helpful?

I checked the 3 settings for Firefox 11 and Firefox 12. They are 2, 2 & -1 for both.

When I go back to Firefox 11 the program runs.

I have been running this compiled program since Firefox 3.

The program is self contained in a folder. There is no install.

The program takes over the keyboard. Option F8 kills program before execution ends.

The purpose of program is to generate an email to Registrar to remove spamming domain based on domain inputted.

The email is put in the Outbox (any evidence can be added here to email).

Program does a WhoIs and gets the Registrar email addresses from table in program folder.

Size of zipped folder is 1.4 MB

Name of program is Complainterator (small distribution)

I found the website for the program and its support forums, but not enough information to understand exactly how it generates a new Gmail message.

If you look at your full browser history (Ctrl+Shift+h or History > Show All History), can you see whether there is any duplication of the URLs the program is attempting to launch in Firefox? I'm thinking perhaps something might have changed in loading external URLs that are the same as already-open URLs.

My MAILTO problem was finally fixed on the June 29, 2012 Firefox version 14 beta 10 release.