Let's track compatibility issues for OS X 10.9 Mavericks here.

I find a bug with mavericks DP2 only. I can reproduce it with v22, v23 beta an aurora. If I set to always ask where to download. First time I download a file it shows finder window to select where to download. If in the same session (don't close FF) I try to download another file, FF never shows finder window and the menu only shows Firefox/File/Edit (no other items). If I look in OS X console, it shows this error message : 29/06/2013 17:24:15,887 firefox[249]: Mozilla has caught an Obj-C exception [NSUnknownKeyException: [<FI_TTaggingSuggestionsTableViewItem 0x10b492540> valueForUndefinedKey:]: this class is not key value coding-compliant for the key image.] (0x10b492540, change every time). If I close FF, reopen it again and try to download again, it works. I had no problem with DP1

Dave, this bug is a meta bug. You hit bug 888605.

For reference, here are the (some?) changes between 10.8 and 10.9: AppKit changelog https://developer.apple.com/library/mac/releasenotes/AppKit/RN-AppKit/index.html#//apple_ref/doc/uid/TP30000741 AppKit API diff: https://developer.apple.com/library/mac/releasenotes/General/APIDiffsMacOSX10_9/AppKit.html Foundation changelog: https://developer.apple.com/library/mac/releasenotes/Foundation/RN-Foundation/index.html#//apple_ref/doc/uid/TP30000742