Smaller devices (such as netbook and notepad computers) often have a screen resolution of 1024 x 600 or less. Thunderbird is not yet optimized to display at this resolution. Some windows (such as the Account Settings dialog) are only partially displayed. You may not be able to see all the fields and buttons on these windows. Typically, the lower portion of the dialog is off the screen.
To work around this problem, you want to grab any of the corner of the window and resize it. If you are unable to see any corner, install the Theme Font & Size Changer add-on, set a smaller font, resize the window to be much less tall (or wide), then set the font back to a usable size.
Not all window types can be resized. If you cannot grab a window corner, it is not possible to resize it.