The real problem for Firefox is not the interface changes that people like you whine about, it's mobile. Now 30% of traffic is mobile and Firefox doesn't have an app for any Apple mobile devices and is effectively excluded from Android by Google's Microsoft-like illegal anti-competitive licensing deals with manufacturers (you can get the app, but it's not preloaded and only a few geeks ever would). They're also up against a massive advertising campaign, with every Google property having a huge pop-up like ad telling users to use Chrome. Chrome users don't see this, but Google is doing everything they can except adding the words "or else".
Mozilla is doing a great job with Firefox, but they are up against a billions-dollar corporation that has set its sights on owning all the means to access the web that is spending hundreds of millions of dollars a year and is willing to break the fair competition laws to do so.