I have Portal 2.6 ALPHA2 (bundled) and am getting some strange behavior with Firefox.
I have added an instance of the Forums Portlet, and created a new page with the portlet in the center column. (By the way, can the left column be moved to the left of center instead of to the right? Very confusing and counter-intuitive)
It then displays the forums halfway down the page in Firefox, but OK in IE7.
If I add a new portlet in the left column, all is OK in IE7, but now in Firefox the new portlet is now on top displayed full width, and the forums are below, also full width.