Comment by oldtapwater

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)

1. https://addons.mozilla.org/en-US/firefox/addon/delaywebpage/ 2. https://github.com/OskarDamkjaer/FirefoxDelayWebpage