So as a silly example, that means about:mozilla or Chrome's dinosaur game shouldn't have been shipped at all?
Easter eggs started at a time when software and games were shipped on cartridges, disks, or other media. If there was extra room available, it didn't affect the user experience to include them (performance was always very much in mind back then).
Today, however, we see a couple of changes:
First, apps and games are mostly downloaded. Easter eggs take up additional space, slowing down your download in the best case, and eating away at data caps in the worst cases. So you're already doing something that is somewhat hostile to the user.
Second, both Mozilla and Google position their browsers as fast and lightweight. Does including Easter eggs actually help further either of those claims? Most likely not. One Easter egg might not really impact performance at all, but at what point does it become a problem? There are probably different teams working on the browser, and if each of them are adding Easter eggs, how long before those start to affect performance, whether because it's cruft or because many users are on older systems that don't have as much memory as the typical developer is used to?
This at least bears consideration.
It did not do anything, unless you manually switched on an about:config value. And if you did, then all it did, was flip some random words on webpages upside down.
No, I don't like it either to have eastereggs in my software that wasn't just put there for the fun of it, and rather was also motivated to some degree by a continued commercial relationship. But if this commercially motivated easter egg helps to gather more money to improve the software, which itself is non-commercial, and is as harmless as Looking Glass, then I do not see a problem with that.
And let me repeat that, if it is as harmless as Looking Glass. I do not see a reason to categorically exclude any sort of commercially motivated thing from the browser. Even including some actual ads would in my opinion not be unthinkable, given that they get enough money for it and have effective ways to do good with that money, while especially also taking into account that users will get pissed off by it and leave the browser, effectively slimming the ability of Mozilla to do good.
If you take everything into account, you can be morally on the good side without having to resort to never doing things from certain categories.