Hello, Firefox on Linux tries to match your GTK theme, which is Mint-X, this is why the colors appear to be different, it is attempted to blend in with your GTK desktop environment. If this helped please mark as solution!

Firefox on Linux has required GTK 3.4 (three.four) at minimum to run since Firefox 46.0 which means you need to use a GTK3 theme on OS.

The plain scrollbar and no scrollbar arrows looks like you have a GTK2 theme in use on your Linux. It is still mostly usuable with a GTK2 theme in use.