#1, Listen to the users, even if it makes the developers lives harder and the code base uglier. (although the firefox build system is just sad, and is a symptom of the entire project, "make" should actually build a working browser)
Stop fsking with the UI and using creative non native looking stuff just to be cool like chrome, and instead focus on making the rendering/JS/developer tools engine best in class. Along with, stop breaking shit. Hiding shit in about:config and then silently removing the option doesn't make users happy, if they spent the time to figure out how to disable search in the address bar because they are tired of accidentally telling google/etc where they are browsing than actually honor that setting, or better yet, give them that option in the config UI rather than pretending they are all idiots and don't understand how computers work.
There are too many chrome only web sites, so make the developers happy with tools that make their jobs easier. About:memory is better these days, but its still a far cry from what it could be, and AFAIK its still doesn't have something similar for CPU or networking outside of the network and cpu tracing functions in the developer tools. I want to be able to manage my browser with similar functionality to my OS (aka what tab is sending/reading all this data, then drill into what/where its sending it along with better whitelist/blacklist functionality/etc)
Then for users, you will gain their appreciation if it feels faster than chrome, which far to often is still false (despite it too getting better). And yes, for tabs, menus and the like using the native widgets not only will make people happier when they change their system color schemes and firefox isn't doing its own thing, but the system components are frequently far far faster to render than firefox's. And yes, sometimes the code to have multiple UI toolkits is ugly, as is the code to support optimizing some JS path, deal with it, thats the job.
I could go on, but others have said some of my other points.