The present invention relates to salt forms of the Fibroblast Growth Factor Receptors (FGFR) inhibitor 3-(2,6-difluoro-3,5-dimethoxyphenyl)-1-ethyl-8-(morpholin-4-ylmethyl)-1,3,4,7-tetrahydro-2H-pyrrolo[3′,2′:5,6]pyrido[4,3-d]pyrimidin-2-one, including methods of preparation thereof, where the compound is useful in the treatment of FGFR mediated diseases such as cancer.