That is the new look of the print dialog which is created in the new versions of Firefox which might have been a window in older versions. It looks to me like it is having trouble loading. You could try starting firefox in safe mode by pressing Win+R -> `firefox /safe-mode` to see if it is an addon related problem. If that does not work then I would reinstall Firefox or install Firefox Nightly just to see if that changes anything. Something like this could even be related to a system update and possibly a restore point could be needed.