This may fall into the category of "so obvious I'm the only one who didn't get it", but I just had to share it. I'm preparing a blog post where I've got an intentional error in my JavaScript code. I noticed that it wasn't showing up in the Firefox console though. I added a console.log message before the error - and it showed up - but my error never did.
Check out the screen shot below:

Notice that CSS warnings are showing up - which is cool - and "1" is my console.log message. But my JavaScript error isn't there. I clicked the console drop down to see what options were there:

Yep - logging of errors is enabled. So what the heck? Turns out I had to also check the options on the JavaScript tab:

Ugh. I can't imagine why Errors would be turned off, but maybe it's a sensible default for some reason. That complaint aside - I have to say I really like the look of the console.
Archived Comments
Check out the 3D View and Responsive Design Mode accessible from the top right menu icons.
All very impressive!!
I've seen both. I like the responsive design mode. The 3D View seems... cool but impractical. But I haven't actually used it for 'real' yet so I may be wrong.
As just an FYI, with a virgin install of FF on a Win8 VM, this defaulted to on. But I figure this blog post will be helpful for folks who - like me - somehow got it disabled it in the past.
@Raymond:
The 3D view comes in really handy when you're trying to troubleshoot z-index issues. That's about the only practical use I've found for it, but it really shines there.