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I don't think this has anything to do with multiple browsers, as is suggested in that thread. I'll quote myself here:
I was going to say the same thing. You're clicking in the wrong place!
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The correct option is "Watchlist." It's a wonderful feature that, when used, allows you to accurately follow all the topics you want.

Great Scott!
It only took me : 667 days, 15 hours and 56 minutes to figure that out!
Life changing experience.
I probably never clicked “watchlist”, while I was frantically editing the “watchlist” list of threads, without noticing any effect in the “Show new replies to your posts” list.
I guess it not super intuitive, but not going to argue on this pretty effective solution.
Case solved.