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?
Now, the option to remove it from about:config has itself been removed.
I believe that the icon was quite enough to distinguish visually between Private and normal windows. The text is just superfluous, and it takes space in the tabs bar so that less tabs can fit in.
I don't think the option in about:config should be restored... i respectfully believe the change itself should be reversed!
The icon in the title bar is enough. The text just uses up valuable tab space, especially when you have two browser windows arranged side-by-side (horizontal split).
If needed, the "New Private Tab" content could be adjusted to more clearly indicate that the current window is for private browsing or explain what this means.