Thread:Support/Preference choice not being respected

I have "Enable collapsing of items in the sidebar in Vector skin" unchecked in my preferences (and, of course, the Vector skin selected), but I'm still having to click-open collapsed menus (e.g., Toolbox) in the left margin after logging in. Interestingly, while a page is loading, the items will briefly look like (I assume) they should: a "flat" list. But as the page loading completes, the "hide/show" arrows appear (and, if I haven't expanded the Toolbox menu since logging in, it will be collapsed). I'm on Firefox 10.0.1. Anyone know what's going on? I just created an account here, so I'm not familiar with the behavior of this particular wiki, but all other MediaWiki wikis I use regularly respect my choice to not "enable collapsing".