They have a duty to moderate public-facing systems. This is a link/bookmark sharing system, so obviously bookmarks that are pointing to things that are illegal are going to be dealt with.
I'm pretty sure there are reasonable expectations by governments that you maintain adequate moderation, especially the EU. If Mozilla were to deploy a public facing system and not moderate it, they would catch hell from the EU and likely be fined out the ass.
If you're going to store something on someone else's computer (Google cloud), they have every right to control what is and is not allowed on their systems. Don't like it? Use encryption, selfhost, etc...