While it is possible to use text-align: left or right and actually mean it, generally what you mean is start or end. There used to be magic behavior for some things, which treated left as start instead of left, prior to bug 299837 adding support for end and taking away the magic, so we may well have regressed rtl in a few places between 2.0 and 3.0. (We may also just have bogus rules in bogus selectors: it doesn't matter where the text is aligned for #folderUnreadCol since we don't have one anymore.)
The title is a bit confusing, perhaps you should make it something like: "Should use text-align: start/end instead of left/right in most places".
Summary: text-align: left or text-align: right is rarely right → Should use text-align: start/end instead of left/right in most places
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