When your computer calls a file like a PDF to your default browser it does so with a command that looks something like this: 

yourBrowser -url "file:///c:your.pdf

When using getUrl();  to say:
getUrl("your.pdf","_blank", "GET");
flash actually posts a command like this:

yourBrowser -url "file:///c|your.pdf

Note the Red. Flash changes the ":" to a "|"

Now for what ever reason, IE automaticly interperets them both as a semi colon or ":" But Firefox seems to interperet the "|" as a new tab. 

So what do we do?  Well we are going to create our own URL, convert it to a string and replace the "|" with a ":"

The first step of this is to find out the directory our SWF resides in and turn that into a variable.

var swfDir:String = _root._url;

next we find the last slash in that string:

var lastSlash:Number = swfDir.lastIndexOf("/");

then we find the pipe or "|":

var colonSpot:Number = swfDir.indexOf("|");

Now we are going to create and empty string and piece all that stuff together replacing the "|" with a ":".

var myUrl:String;
/*that means if we found a "|" if we didn't the colonSpot value would be -1*/
myUrl = swfDir.substring(0,colonSpot);
/*that makes our Url var = everything BEFORE the "|"*/
myUrl +=":"
/* that ads the ":" */
myUrl+= swfDir.substring(conolSpot+1, lastSlash+1);
/* now we just added back everything after the | */

ok Now we write our getUrl() with our new variable "myUrl"

AS 2.0

myButton.onRelease = function()
    var targetUrl:String = myUrl + "yourfile.pdf";
    getURL(targetUrl, "_blank");

AS 3.0

function newGetUrl(event:MouseEvent):void{
var targetUrl:URLRequest = new URLRequest(myUrl + "myfile.pdf");
navigateToURL(targetUrl, "_blank");
myButton.addEventListener(MouseEvent.CLICK, newGetUrl);

That’s it, another bug work around.