At least Firefox can be altered to become what you want it to be because Firefox respect's a users software freedom. Far more important than vagaries like "fast" and "not bloated" is how a program treats its users. Proprietary browsers leave users no opportunity for improving the program. Thus security issues in proprietary programs go unfixed and are exploited for years. This, in turn, allows others to invade people's computers and leaves users helpless. This is exactly what happened with Apple's iTunes for over 3 years. I would not be surprised to learn that software proprietors including Microsoft, Google, and Apple are doing similar things with proprietary web browser programs as well.
So while I like trustworthy programs like other computer users, I know that I can't ascertain the trustworthiness of proprietary programs like Microsoft's Internet Explorer, Apple's Safari, and Google's Chrome. The extent to which any of them are built from software that respects my software freedom is irrelevant because proprietary programs and their updates are essentially black boxes. I can't possibly inspect or fix all of the software I use, but I can put myself in a position where I stand to benefit from the improvements a lot of programmers make by exclusively running software that respects my freedom to run, inspect, share, and modify—free software—freedoms I value in their own right.