When using WordPress version 2.5.1 (Debian Lenny package) it is not possible to add/upload media using the “Add media” button in the editor. If you click on Add an image, a new window appears where you should have the option buttons “Browse” and “Upload”, but they’re gone. What’s left is a “Choose files to upload” button that won’t do anything.
I have experienced this problem when using Mozilla Firefox 3.0.14 on Linux with the Shockwave Flash add-on enabled.
Workaround: When this add-on is disabled the add media problem will not occur. To disable the add-on, open Firefox and go to ‘Tools’ -> ‘Add-ons’ and select disable Shockwave Flash.