Created attachment 317721 [details] screenshot I've tried uploading a new icon for Nightly Tester Tools a couple of times but it comes out as a garbled mess
I've managed to re-save my png file with Pixelmator and AMO appears to have accepted that version so there is something confusing it about the output from the Gimp.
Just tested and this is still the case in 3.5.
Maybe this has something to do with bug 373675 and/or that this icon is "8-bit gray+alpha, non-interlaced" - maybe that is contributing to the fail.
Forward-duping due to the more useful summary.