Why Firefox was successful in the first place Firefox was created as an outgrowth of the Mozilla-suite that was smaller, lighter and faster because many components that were not really needed were left out.
What makes Firefox special What sets Firefox apart from all other browsers is the huge wealth of available extensions which allow for almost any functionality that can be imagined for a web browser (and even some things beyound that) and allows for great configurability and advanced features without bogging down the default install.
What Firefox could have been Firefox could have become the rock solid browser that "just works". The excellent extension-system would make it possible to have a basic version that receives almost no changes except for bugfixes and optimizations while all the experimental stuff and new functionality is done in extensions. As Firefox' own story shows, most people prefer a lean and fast browser to a bloated do-it-all monster. By now, most web standards have stabilized and the rate of new features needed in a web browser is diminishing. That means that we should have less releases and less changes.
What management has decided instead But no. Management decided that Firefox needs a extremely rapid release-cycle (like Chrome), automatic updates (like Chrome) and meaningless marketing-driven version numbers (like Chrome). Of course Firefox will never really be like Chrome because Chrome's advantages (like independent tabs) would require a complete reimplementation. Also Chrome is still a young browser where a rapid release-cylce may make sense while Firefox looks back at more than a decade of development.
Combining the disadvantages While combining advantages of two products is usually hard work and sometimes even impossible, destruction is usually much easier and faster. When you combine the release-policy of Chrome with Firefox, you gain not a single advantage, but the extension-system no longer works reliably, because when Firefox gets updated from a major revision to the next, all extensions have to be retested and released anew. Therefore a lot of things break even when Firefox-management refuses to admit it.
Management refuses to listen Search for "downgrade Firefox" on the Internet and you will see literally thousands of people complaining about things that a (usually needless) update broke. Most of the time the problem are extensions that no longer work, but quite often also new bugs are introduced by new or changed features. In other words, Firefox management has decided to sacrifice the most important feature of Firefox for marketing-driven version numbers.
Firefox-management should understand that the 1990s are over and people are no longer buying a new computer every 2 years and upgrade their software even more often. There have been no radical changes in web standards for a long time and there will be even less changes in the future. The new features and especially the new usability-features (all of them) are not needed in the default install. Changes in the user-interface could be tested using extensions and maybe integrated after a couple of years if found reliable, useful and wanted, but there is absolutely no reason to change anything just for change's sake. In the meantime, many old bugs remain unsolved.
What we need is at least one browser that aims at creating a bug-free browser instead of a perpetual usability experiment.
Management can be fired The good news is that Firefox is free software, which means that Firefox management does not own the codebase, but only the name "Firefox" (plus artwork, etc.). Firefox itself was started as an offshoot from the Mozilla-suite and a group of people who do not refuse to listen to their users may just create another offshoot from Firefox (which is called "fork").
For example the "IceCat" browser from the debian project (which is a Linux-distribution) is a rebranded version of Firefox with some minor changes which could grow into a real fork, if the IceCat-developers want (and also put out a version for Windows). Given the fact that the debian-project is famous for focusing on stability and reliablility, such a development would certainly fit into their general strategy and would make a lot of sense in my opinion.
Chance is now better than ever Because of the recent management-decisions the chances for a new fork are in my opinion now much better than 2002, when Firefox itself was created as a fork. Firefox basically offered not a single new feature compared to the Mozilla-suite, on the contrary it offered a lot less - to reduce the bloat was the whole point of Firefox. A new fork optimized on reliability and stability could repeat the same story. Basically this version could become the bugfix for the "my extensions no longer work"-problem that has been popping up so often lately.