my point is that they are different things for different purposes, therefore one cannot be better.
you can say that firebug is better than chrome inspector, or that an old honda civic is better than an old ford escort, but non-apple-to-apple comparisons are inherently meaningless.
even if the two accomplished all the same things, your question would be like "whats better, shell or gui", which is a long-debated matter of opinion.
the command line is a shortcut utility for launching common tasks, firebug is an interactive debugging and performance evaluation suite.
while there are some debugging tools in firefox's new command line, that's not its primary focus.
if i could only have one, that's easy; obviously firebug has a lot more features.