Firefox can put multiple versions of an image on the clipboard. From
tests, there's always a fairly useless JPG, and sometimes a decent
PNG, but of the formats it advertises, in my tests, only these two
are present. It seems a bit timing dependent, in that if Krita is running,
chances for getting the PNG clip are a bit better, but still not 100%.

There's nothing more we can do that check whether we've got the best
image type, in the order PNG, TIFF, BMP, JPEG, and use that.

This is debug output that shows what's on the clipboard if we're lucky:

Format "TIMESTAMP"
Format "TARGETS"
Format "MULTIPLE"
Format "SAVE_TARGETS"
Format "text/html"
Format "text/_moz_htmlinfo"
Format "text/_moz_htmlcontext"
Format "image/png"
data 290949 mime "image/png"
Loaded image succesfully QImage::Format_ARGB32 true
Format "image/bmp"
data 0 mime "application/x-zerosize"
Could not load image of type "image/bmp"
Format "image/x-bmp"
data 0 mime "application/x-zerosize"
Could not load image of type "image/x-bmp"
Format "image/x-MS-bmp"
data 0 mime "application/x-zerosize"
Could not load image of type "image/x-MS-bmp"
Format "image/x-icon"
data 0 mime "application/x-zerosize"
Could not load image of type "image/x-icon"
Format "image/x-ico"
data 0 mime "application/x-zerosize"
Could not load image of type "image/x-ico"
Format "image/x-win-bitmap"
data 0 mime "application/x-zerosize"
Could not load image of type "image/x-win-bitmap"
Format "image/vnd.microsoft.icon"
data 0 mime "application/x-zerosize"
Could not load image of type "image/vnd.microsoft.icon"
Format "application/ico"
Format "image/ico"
data 0 mime "application/x-zerosize"
Could not load image of type "image/ico"
Format "image/icon"
data 0 mime "application/x-zerosize"
Could not load image of type "image/icon"
Format "text/ico"
Format "image/jpeg"
data 33290 mime "image/jpeg"
Loaded image succesfully QImage::Format_RGB32 false
Format "image/tiff"
data 0 mime "application/x-zerosize"
Could not load image of type "image/tiff"
Format "application/x-qt-image"
Final QImage QImage::Format_ARGB32 true

We cannot do more than this, so I'm closing

BUG:406781