Add 'Save as JPG' to 'Take a Screenshot' feature in Responsive Design Mode

Hi,

Can you please add the ability to save as JPG to the ‘Take a screenshot’ feature in Responsive Design Mode? Right now it automatically saves as a PNG but I need them as JPGs and would be easier than converting afterwards. Most screen capture extensions offer this ability (to choose file type).

Filed this bug: https://bugzilla.mozilla.org/show_bug.cgi?id=1421552