Very nice extension! Sometimes the first image on a page is briefly displayed in color, maybe it's my computer (Firefox under Linux). Is something like this also possible for Android?
and
androids do usually have a greyscale mode on the devices preinstalled, I can work around a bit to make it compatible with firefox android as well but I think an app is what you are looking for? greyscaling particular apps right?
Greyscale mode is an option I'm using right now.