Problem with Microsoft Outlook in Firefox

I have a Microsoft Outlook account, which I access through both Firefox Portable and Internet Explorer. When I use the former the commands in the coloured band are half-visible because the bottom half of the white lettering would be located outside this band (against a white background). When I use the latter, I do not experience such a problem.

I am using the latest version of Firefox Portable in a laptop the OS of which is Win7 Ultimate 64-bit.

Thanks and Regards

Paul