I checked your gallery in IE, and can confirm your custom button is not working. However, I noticed you have a mistake in your original code:
getURL('asfunction:_root.link,#/content/welcome/');
When using this ASfunction method, you do not use the # character, as this is specifically for the SWFAddress method.
Also, looking this over again, it occurred to me that I don't think you can use this for getURL. The flash-native ASfunction is meant to be triggered from text links in textfields (<a> tags).
For my original examples of this code, I have:
<a href="asfunction:_root.link,content/folder/">link</a>
<a href="asfunction:_root.link,content/folder/image.jpg/">link</a>
So for your button, instead of using the getURL command, please simply try this in your button:
_root.link("content/welcome/");
Optionally, you could create the link from a textfield html tag and use the same as in my original example.
I think this should sort it out!