Hello,

If you set Firefox to always start in a private browsing i think the button color wont change.

To check go to Tools >> Options >> Privacy if you seted to Never remember history this mean that Firefox will always open in private browsing then your button wont change of color

Also, you can use Private Browsing in a tab now:

• Private Tab

Also:

Private Browsing

If you start Firefox in permanent Private Browsing mode then you can't leave Private Browsing mode without restarting Firefox and all tabs and windows that you open are automatically in Private Browsing mode, so there is no need to have the purple mask visible.