In case it's already light, but looks a bit unclean then you might have set it to Firebug theme which apparently comes with Firebug. For some reason this was the default for me.

Thanks. As someone who isn't even a Firefox user I would have never found this. The dark UI was tough for me to read.

The Dark UI looks really cool, but switching to the light theme feels so much better. It suddenly feels like Chrome except with better dev tools!

Either way, the new UI changes in this edition make it way more elegant and appealing.

Thank you so much. I felt like I was in a teenager's bedroom.

In dark theme, the contrast to the Yosemite system toolbar is terrible and the tab text is extremely hard to read.