Ubuntu LTS as a guest OS: some issues (Virtualization)

[LinuxIsOne]

Hi,

In of the PC Ubuntu LTS has been installed in the virtual box but the
strange issue is as follows:

While screen resolution is not set Ubuntu is seen only in the center
position of the whole monitor, while when I open any application, it
opens with the default size, so more bigger while the visualization is
so short. The current resolution is:  800 x 600 and there is not an
option bigger than this in the Monitor Preferences. See this:
http://postimage.org/image/3rg3gw3eh/ the whole of the opened firefox
fits itself in the entire way, and still I am not able to see the
complete window of FF. How can I fix this?

Thanks.