@Akash Vyas Hi ! I would also prefer the solution of an extension. I'm already using a multi-language keyboard (fr/en/cn) and it would be a hassle to have to alternate between 2 keyboards.
@Anshu kumar Said there would be a search about an alternative method in 2016 but there is currently still nothing supporting Firefox Android Except Keyboard.
Would the Dev/Adaptation of one more Extension cost this much efforts (You have already five to maintain) ?