I know firefox 3.0b5 is not yet and probably not tested...
but here is what I have when browsing using firefox 3.0b5 under my ubuntu:

just for info...
As of the official release of Ubuntu 8.04 LTS using Firefox 3.0 Beta 5, the problem you are mentioning doesn't seem to be there anymore. I've seen the issue using Beta version of Ubuntu with Firefox 3.0 Beta 5. The Ubuntu team might have been using a debug enabled version of the browser in previous versions.
As of the official release of Ubuntu 8.04 LTS using Firefox 3.0 Beta 5, the problem you are mentioning doesn't seem to be there anymore. I've seen the issue using Beta version of Ubuntu with Firefox 3.0 Beta 5. The Ubuntu team might have been using a debug enabled version of the browser in previous versions.