If you use a not standard theme then try the default theme (Tools > Add-ons > Themes).

See Troubleshoot extensions, themes and hardware acceleration issues to solve common Firefox problems

If you want to test that beta version, you should create a new Profile specifically for 4.0 beta so you don't carry forward your existing extensions and themes, which are what's probably causing that glitch in the User Interface. It looks like every extension is going to need to be re-worked for 4.0, and as usual with a new major version every theme is liable to break something in 4.0.

If you aren't prepared for small glitches like that, don't use a pre-release version like Firefox 4.0 beta. It's labeled as beta because it's not a finished product, and won't be ready for a public release for another 5 to 6 months.