Firefox Nightly used to return to the last geometry settings (size and location) when you do a restart of the browser.
Currently, FFN chooses to ignore the last settings and fails to reset properly.
This is with multiple instances of the browser. This has been tested with up to 6 instances. The first instance seems to work somewhat correctly but the subsequent instances do not... which leaves the other instances with seemingly random geometries (random but somewhat predictable in nature).
George...