Yeah it appears on chrome as well. There isn't any evidence that it's purposely "slowing down" anything. I had a quick glance at the Reddit thread (been avoiding it as much as possible, but had to visit in incognito to confirm the source for this outrage) and it looks like it's part of a small script to check if an adblocker is present and disabling video ads from playing.

It's possible FF have a delay in playing that first video, but also the test methodology isn't super reliable because of caching.