A very nice addition of a race to any eastern or Asian themed campaign setting, or even just a hidden race to apply in any campaign. The stats are pretty balanced, especially since the alternative form doesn't seem to give any ability bonuses.
There are a couple things about the race that needs clarification however. One downside is no combat stats are given for their fox form, such as their AC (I assume armor melds into the form and is inactive until they assume human form) or if they have a natural attack in that form. There's also a vagueness in the
Master of Disguise feature. Do you take the form of the same humanoid each time with whatever minor alterations you desire? Can you shapeshift into any other individual? I'd say it should instead allow you to assume your specific humanoid form AND give Advantage on Disguise checks to appear as another humanoid form through magical alterations or proficiency with a disguise kit.
Aside from the slight vagueness, it's a very good race to bring in if you like the flavor, which is also provided and fits the pop-culture idea of kitsune. I'd go a step further with the fluff and lore by saying you're not limited to human form, but whatever race you grew up with (elf kitsune, gnome kitsune, even orc kitsune if you want) by just taking on the physical traits you liked in others as you matured.
[5 of 5 Stars!]