Problem with browser window resolution

[ergo]

Hello,

first some of system details: Host: OS X; Guest Ubuntu Zesty; Laptop Display 13“.
Guest Additions are installed. VB and the additions are uptodate.

My problem:
For some reasons I want to have a screen resolution of 1366x768 for the guest firefox browser window in VB.
Since some days (after Firefox update to V. 55.xx) while all other optional screen resolutions of virtualbox menu work correctly, the resolution above does not. Only 1360x768 is displayed when setting 1366x768 resolution in Virtualbox.
I tried the command in the terminal „VBoxManage controlvm XubiSurf setvideomodehint 1366 768 24“ and played a little bit with changing the width resolution from 1366 to 1368 or 1370 using this command line.
But this did not hit the nail, because the resolution of the firefox window jumped now from 1360 to 1368 according to a testing website, but did not hit 1366, while all other virtualbox resolutions are displayed exactly as chosen.
Apparently there seems to be a direct take over of the VB display settings and the settings for firefox in the xulstore.json file seem to be ignored instead.

Any ideas how to fix the width resolution 1366?

Thanks in advance.

[socratis]

Any ideas how to fix the width resolution 1366?

Resize the window to 1366 by hand??? Honestly, I can't understand why you'd think that VirtualBox has absolutely anything to do with an application running in the guest... (hint: it has nothing to do with it).