You mean besides that time Firefox reverted everyone's search engine to Yahoo?
The above experience is on the developer edition of firefox, as I haven't tried it on the normal stable release.
(I'm on Mozilla's side in this discussion, but that particular move was terrible, even if not too consequential, in my opinion.)
It's pretty normal for a program installer to set itself to handle files.
OS shouldn't set your browser.
Let me lay out the analogy more explicitly. I install Firefox, and specifically choose for it not to be my default. I later download an updated version and choose default settings in the installation. Firefox then sets itself as the default browser; it overwrites my previous choice in favor of the default.
I install Windows. I specifically don't set explorer as the default. I install an updated version of Windows and choose default settings. Windows overrides my previous choice.
Firefox will only change things if you specifically get a Firefox installer, where it thinking that's your intent is understandable.