"Web Extension API" is exactly the issue at stake here. Chrome develops a new API for extensions. Everyone jumps on it. Firefox abandons it's powerful (although insecure) extension API, and re-writes it to follow Chrome's. There's a lot of marketing-speak around the reasons FF did that, but it really boils down to: Chrome is winning in the extension ecosystem, let's make them all compatible with FF too. At the loss of the entire existing extension catalog.