OK, here’s an update of where I have got to looking into this. From a fresh installation of both WP and Mingle I have created a home page slideshow using two 990px x 300px images. At this stage the top navigtation menus are defined but neither of them actually has any menu items in them. Result: slideshow works fine and zooming out in Firefox is not a problem.
But…..
I then create one menu item for Home and give it the homepage URL. This produces a single “current menu item” tab in the left-hand navigation menu. Result: zooming out in Firefox makes the slideshow jump to the right.
What I would like please is for someone to repeat this process and tell us if the bug is reproducible. If it is, then I hope @andy will be able to address this issue as soon as time permits.
I have determined that what is pushing the slideshow to the right is the left nav bar container when it has a “current menu item” tab in it. I think I am right about that, but if someone reading this knows differently please speak up.