Thread:Project:Support desk/Sidebar floats underneath articles

While getting to fill my newly created wiki with content I noticed, that the sidebar floats underneath the articles, or, in other words, the navigation-links in the left section start just right underneath the white box of the article in the middle section of the screen...

I tried to find an answer to this issue on the web - but was not successful! Perhaps it may be a new Firefox-issue? Or I screwes up any settings?

It only happens with Firefox (I am currently using version 9.0.1), not with InternetExplorer...

The effect described is to be viewed in the monobook-skin (default).

Versions-information:

MediaWiki: 1.15.5-2squeeze1

PHP: 5.3.3-7+squeeze3 (apache2handler)

MySQL: 5.1.49-3