I think Firefox lost users to Chrome mainly because Google advertises their product constantly.

What you point out about trying to imitate Chrome is right though; when they dumbed down the very complete and functional settings window into an almost empty HTML page that forced to resort to about:config to do anything non trivial, or when they literally copied the terrible Chrome main window structure, I literally wanted to puke. I'm not sure however this is the reason why normal users distanced from the product. Firefox back then was the alternative to Internet Explorer and its load of security and privacy problems, so it started in a context where a good number of users actually wanted to switch to a different browser, despite some lock-ins such as Active-X and the like, I believe even Opera had some well deserved success at a moment for that reason, but none of them could compete anymore when the new kid in the block was a strongly and pervasively advertised product like Chrome. Firefox today is faster and more user privacy friendly than Chrome; to me if most people don't use it is simply because they don't know. Small personal example: of the many people I installed it, the number of those that went back to Chrome is a single digit percentage, which should suggest they didn't choose Chrome initially, but rather they didn't know about the alternatives.

Now it's the time for all Firefox users to tell others that they chose Firefox, and why, then for Mozilla to invest some damn money in advertising!