Well, I wouldn't if I were Mozilla because C++ has a larger user base and I could count on more people contributing to the code. Not to mention the fact Mozilla background and history is based on C and C++. It would be silly of me to pretend that much.
However, say, a browser fully developed in Ruby capable of rivaling Firefox wouldn't shock me. The gecko engine is not a sublime piece of engineering. Far from that. I would wager Presto to be a superior engine (not to mention it's the only of the current 4 big ones that passes the Acid2 tests). And even if a browser/engine developed in Ruby was slower than Mozilla, but far more standards complaint and offering a richer functionality, wouldn't that be better?
Again, ask yourself: How slow is slow? Say, 2 seconds extra, average, taking to load a page is unacceptable? Maybe for some. But it's not a reason to dismiss this language as a potential tool for just that if it happens I'm more proficient at it.
Count two seconds in your head. Does that time span shock you? What if I told you the next computer processor would reduce that to just 1 second? And the next one to half a second?
Well, you should always learn more than one programming language. It's a matter of common sense. Even C or C++ aren't the best choices for some programming tasks. It will also give you more power over problem solving since you'll be the proud owner of more than one programming tool.Well, I am mostly interested in applications for end users.
As you well put it, it annoys you the fact C++ seems to overly complicate certain programming tasks, like that of developing for a GUI. It's not its fault really... it's the GUI API that complicates your life. But aside from that, what you should look at is spreading your knowledge through a few different tools and use the one that suits you best at the time.
If you are developing a universal measurements converter, does it really matter if Ruby is slower than C++? Other things will instead come to your mind, like how fast can I do this in C++ or in Ruby? What language do I feel more comfortable programming with? Which one offers the least maintenance cost?