The decline of Firefox started with Chrome's JavaScript engine, who was years ahead of Firefox's performance. The focus on JavaScript-heavy websites was already

The decline of Firefox started with Chrome's JavaScript engine, who was years ahead of Firefox's performance. The focus on JavaScript-heavy websites was already growing, and Firefox was slow to catch up. Now they're about the same performance-wise, but the mind share lost was brutal. there was a period that the only advantages Firefox had over Chrome were memory use and extensions, and they had to get rid of the NPAPI extensions for security reasons.