Works efficiently is step 10000.
x86 is at step 10000, ARM at step 5000, power is at step 0.
Firefox "worked" before this post on power. Now somebody put enough effort to actually make it usable.
The fact that you don't see people complaining about Firefox PowerPC performance on Linux is not because performance was good - it was unusably slow - but because nobody uses Firefox on Power.
Think about what that means. Think about how many bugs in Firefox are reported _every day_ for x86 and ARM, and how many are reported for PowerPC. Is that also because the PowerPC version has no bugs? (no, it is because nobody uses it, nobody reports them, and nobody fixes them).