Switching tabs moves focus from content are to location bar. This is rather horrible for ux especially when one uses keyboard to switch tabs and tries to scroll the newly selected page.
We're supposed to be remembering per-tab which element has focus, and move focus accordingly. Can you provide more detailed STR as to what you're seeing and how to reproduce?
This happen when there isn't any particular element focused, so the document itself (or body element) should have focus. But, this is not happening using latest Nightly, it seems. When this was happening, any tab switch moved focus to location, if the page didn't have some input element or so focused.