It's not so much your footer is jumping around as it is the content of your clients page. In FF on Windows, the footer appears in the same place (relative to the content) in all but the clients page. On the clients page, you have a lot of space between the content and the footer (looks like the height of the parent box around those two is too tall), and there is too much space to the left of the content area. Once you fix those two problems, it should take care of your footer jumping around.
You should check out the Firefox Web Developer plugin. The outline options can be extremely useful for those kinds of problems.