Why is Firefox 3 breaking with console.log

Question

I have the following:

console.log (a.time_ago() + ' ' + b.time_ago());

This is breaking in FireFox 3, meaning when FF hits that line in the JS, it goes no further. Strangely if I have Firebug open it doesn't break and continues as normal. Some how firebug prevents this issue?

I'm puzzled on this one. Any thoughts as to why console.log would break firefox 3, but not if firebug is open?

Thanks