From the techie perspective, Mozilla didn't have "best years" I think. As a Mosaic/Netscape/FF user, I liked Mozilla for two things, adhering to standards compared to IE and being open source (the second being the reason I still use it today, not because of technology (I don't like tab management with 50+ tabs open e.G, never liked the environment implementation)). I don't consider Firefox a technological marvel, but IE looked very bad. Firefox for me never innovated enough or seemed to have the user in mind. From my perspective the USP was always "We're the open source one" which wasn't enough when Chrome arrived.

Not an Opera user, but from the sidelines it always looked like Opera was innovating more.