One of the things I've liked most about linux (and other *nix systems, such as FreeBSD) is that a system is build up of small programs that you can combine in various ways to get someone that pleases you, the user. That's the unix way. For example, my "desktop" is a combination of a number of programs, including a display manager, window manager, terminal, and file manager. It turns out that I can replace one part (for whatever reason) and get an overall desktop that works in the same way. And it has looked the same for some time. Some might think it reflects an unwillingness to change, others might think it reflects a desire for some consistency and predictability. Take your pick.
This consistency over time is difficult to maintain with the monolithic, graphically orientated programs such as firefox/iceweasel, thunderbird/icedove[1], and the Gimp. This is sad making. What would be nice is if firefox provided basic services, e.g., a first rate rendering engine/Gecko, while making most of the rest of it (e.g., interface) simply a set of addons. That would be the unix way.
[1] For a variety of reasons, I had to move from using mutt and remind to icedove and iceowl. This was over a year ago, and I'm still trying to recover from the shock.
Best wishes,
Bob