67,000 lines of C++ is actually not a lot of code. It really depends on _where_ these lines are (i.e. are they in a hot path? How _often_ they are the executed?) and how well written it is.
EDIT: I know it's been disabled for months - I'm referring to the original disabling of this code.
It says in the first paragraph that TraceMonkey was actually disabled months ago. So it's says more about keeping the codebase clean then anything about memory or speed of the browser.
Removing code from codebases is actually really great for keeping it maintainable. It really should happen more but I get the feeling in a lot of places code is basically never removed. So IMO removing 67k lines of code is good news for the long-term maintainability of any project! (As long as there's some source code left, heh)
EDIT: I know it's been disabled for months - I'm referring to the original disabling of this code.