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?
it takes a bit of time to put filters over website so it is normal to have that but I will see what I can do with it 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?
Your plugin for Firefox Android would be great.
Greyscale mode is an option I'm using right now.