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[Answered] Help with 'selected text' background style
So, the bug that caused this is bug 2058134. The change that was made there is rather inconvenient for user styling, because now the text painting code simply looks if author styles (e.g the web page author) change the selection color and only then use the css specified colors. I'm pretty sure that means that ::selection does absolutely nothing when used in user{Chrome/Content}.css files because they are user, not author styles.
If you want this changed then you should create a new bug and mark it as being regressed by the above bug.