I tried that, but only works for the first visit of a domain? Once I've waited those seconds for the page to load I can mindlessly wander around for hours if I don't close the tab.
You could try DelayWebpage(1) which I made since I couldn't find a good alternative on Firefox. It's pretty basic so if there's any feature missing, feel free to request it, send a PR or fork the repo(2)
One out of the box option is Dev Tools > Network and there's a Throttling dropdown on the right. Set it to something like GPRS and watch the added latency.
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).