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Re:I Add 3-25 Seconds of Latency to Every Site I Visit by fortydegrees on Thursday February 13, 2020 @04:23PM (#22321556)
This applies for all sites though, whereas you want it for specific ones like reddit
Re:I Add 3-25 Seconds of Latency to Every Site I Visit by WorldMaker on Thursday February 13, 2020 @05:39PM (#22322133)
Sure. Still a good place to start if you are looking to trial the idea.

Continuing to think "inside the box" of built-in Firefox tools you could pair it with different windows in different profiles, train yourself to open the "addictive" sites only in the throttled profile. I don't have the Containers add-in installed, but I wonder if you can throttle Containers separately (and if not, might be an interesting feature request).