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windowtype
Type: string
Set to a string which can be used to identify the type of window. This might be used, for example, to distinguish between a browser window and an editor window. Some of Mozilla's window handling functions use this attribute to group windows of the same type together.

Values for window type as found on MXR: http://mxr.mozilla.org/mozilla-release/search?string=windowtype

navigator:browser - Looks like if window has gBrowser it has this window type

devtools:scratchpad - Scratchpad windows

navigator:view-source - The view source windows