It’s a standard timeout function without any context. Most likely thing is that it tries to load an ad and if that doesn’t work in these 5 seconds, then the anti adblock popup is displayed. If you don’t use an adblock, the site loads instantly cause the ad is detected. If you use ublock, you see neither the ad nor the popup, so everything that’s left is a 5 seconds timeout.
While it definitely is shady coding, it’s an anti adblock “feature” caused by incompetent design and not an anti Firefox thing.