[watermelon0]

Most browsers (with the exception of Firefox which has its own store) trust root certificates installed on the OS (at least for Windows/Linux/macOS.)

With mobile devices (iOS/Android), web browsers also trust custom root certificates, but apps have the ability to reject them.

[GekkePrutser]

Firefox on Windows can also be configured to use the system store. Most corporate admins would do this because it makes for only having to manage them in one place. On Mac it can't though, and on Linux there isn't really a definitive system one (unless you consider OpenSSL's).