Flash has an issue with modal browser windows in Internet Explorer 9. When a window is modal, flash refuses to open the file upload “browse” window. It either crashes IE, or waits to open the browse window untill the modal window is closed.
Try the following: Open the Media Browser from an Image field and press the “upload” button. Then press the “Browse…” button and see your Internet Explorer 9 fail to respond:
Sitecore uses a small flash file to mimmick the upload in the “Upload a file” dialog box. Because the Media Browser is a modal window, it stops IE9 from working.
The workaround is to go back to the “Classic” upload mode, by setting Upload.Classic to “true” in the web.config:
<setting name="Upload.Classic" value="true" />
Using classic mode replaces the flash with a normal asp:FileUpload control, and IE9 will survive:
A Quick note: Sitecore does not officially support IE9 unless you are running Sitecore 6.4.1 update 5 or 6.5 update 2. I expect Sitecore to support any newer version from 6.5 very soon.