window.external.AddSearchProvider is part of the OpenSearch 1.1 Draft and is available in both Firefox and IE7/8 but there is a slight difference in how it works, or rather doesn’t work.
Don’t missunderstand me, it works in all of the above browsers but NOT in the same way.
In Firefox you can wrap the function call in another function call, to do object detection and those nice things.
But in IE7/8 you have to call the function from an inline onclick event handler. You can’t wrap it in any way or it won’t work.
If you do you get the most common JS error in IE “The object doesn’t support this property or method”.
Just FYI for those of you who doesn’t want to spend four hours wondering what you did wrong. Like me! ;)