I think that the text does the job better than the mask icon for casual users.
Instead of reverting to the previous style, it would make more sense to add an option in settings to toggle browser.privatebrowsing.enable-new-indicator to false for the people that require it.
Didn't there use to be a Private Browsing icon or indication that lets you know that you're in Private Browsing? All I see is the Regular Firefox icon. Is it something I can enable in Settings?