From what you have mentioned, I don't think you found the right profile folder. The path should include something like "C:\Users\[user name]\AppData\Roaming\Mozilla\Firefox\Profiles\[profile name]".

I've put together some steps describing how to apply custom CSS along with potential pitfalls you may have encountered here: https://imgur.com/a/eXnuFci

If you are still having problems I recommend checking out internet resources like r/FirefoxCSS