Are you on Windows? On Windows, filename capitalization in general does not matter.
But OP seems to be on Linux. I just tried it on Linux with the filename as "userchrome.css" and it was not loaded.

I would've been very surprised, if it did work on Linux, macOS etc., because to my knowledge, as a software engineer, there is no way to implement that without a second filesystem access, which would've delayed startup.
Nowadays, you even have to set a about:config flag, specifically because they don't even want the first filesystem access to try to load the file...