I like Firefox, but…
I remember when their PR people complained that Apple's Software Update could install Safari on Windows if users failed to uncheck a checkbox, while not acknowledging that if you ran Firefox its default would be to take over the default browser role. They're both crimes against your users, but the latter is a much bigger crime towards your users: an installed bit of software can sit forever harmlessly on a novice's computer. Taking over default browser is a forever choice for novices.
It wasn't a big thing, but it forced me to look over some of their earlier decisions and pay more attention to their future decisions. And I came to the conclusion they were more interested in playing with standards than advancing the web. In the case of ambiguous standards, Mozilla would intentionally pick the stupidest possible way to implement something rather than the intended way in the interest of pointing out flaws in the standard rather than providing a good browsing experience.
Eventually I decided I wanted a browser not a political engine. I often have Firefox installed to check for compatibility, but it's the last browser I install.
Safari isn't available anymore on Windows, but I'm pretty sure Firefox will still take over a Mac or PC by default.