Kubrick Tabs Tabs ‘stacking up’

I am running Kubrick Tabs, which is basically the default style (which comes wordpress) modified to have tabs. here is where I got the theme from: kubrick tabs download site

It is quite a nice theme but I am having one problem. The tabs keep stacking up. They start out OK, then if I go to an archive or sometimes view a comment they start to stack up. My theme has 7 tabs: sometimes there is 1 on top and 6 on the bottom, sometimes there are 2 on the first row, 1 second row, 2 third row and 2 on the forth row, sometimes it is normal. Just to be clear, by stack up I mean that the tabs are in several rows, not just one as they should be.

It only seems to happen on firefox, I just checked Explorer, Opera, Safari and they seem to display the tabs as they should be. I am using and seeing the problem on firefox 3.0.1.

I will be making some modifications to the theme, but for now I have the original Kubrick tabs as downloaded installed on my blog here: blog with tabs problem.

It may be a tough one since it only occurs on firefox, but hoping someone can help. I wrote to the author, am waiting. In the meantime, anyone else out there have any ideas? Any one using this theme, any stacking problems? Ideas how to solve?

thanks for your help