Comment 18 for bug 301158

I was able to fix this in Kubuntu. Here are the steps I performed:

1) Opened System Settings
2) Clicked Appearance
3) Clicked on Fonts
4) Under Force Fonts DPI, I selected 96 and clicked Apply.

A notice pops up indicating "Some changes such as anti-aliasing will only affect newly started applications."

Opened up a new Firefox instance and the toolbar fonts look normal now.

Hope this helps....