The sidebar reduces the size of the viewport, and the viewport is what you're centring within. This is obvious and sensible behaviour from Firefox. Please don't centre based on screen res or whatever. Basing your centre position on anything other than the viewport is user-hostile behaviour.
Perhaps bad design, but user-hostile is a bit of a stretch.
Its an entire misunderstanding of how the layout system works.
It's user-hostile enough to make users hostile toward your site, for sure. It's not annoying, it's enraging.
It causes annoying flickering when you resize the window, as it is constantly fighting the browser.