cant change size under headless server (Xvfb)

Binary package hint: seamonkey

Hello,

Today i want set up web snap box using webthumb script (http://www.boutell.com/webthumb/).

This software use Xvfb to generate virtual display, then take shoot of produced image.

Problem, is because seamonkey don't want run in screen size, what i want.

The run seamonkey code is:

first the config:
my $mozillaCommand = "mozilla";
my $mozillaWidth = "1280";
my $mozillaHeight = "1024";

Then, run section
(...)
system("xsetroot -solid $background");
&restartIfNeeded("mozilla", "$mozillaCommand -width $mozillaWidth " .
 "-height $mozillaHeight");
(...)

As you can see, the width and height parameter have 1280 x 1024 dimensions, but i goot about 320 or 640 screen size.

This is example output:
http://grudziecki.pl/upload/4a73398a4cc6bthumb.jpg

(the black screen represent 1280 x 1024 and size of seamonkey, you see will see)

Do you have any idea, why it don't like to be run on dimension what i supported ?

I also try delete /user/.mozilla directory, but it doesn't solve my problem.

I will be great-full for any support. (now i use Windows box, for taking screen-shot) i want as fast is possible, to move for Linux platform.