Firefox 28 erroneously leaves the taskbar visible when switching to fullscreen via F11

0.4.12-RC-46-g317ca41 gcc dbg lin is affected
VBox4.3.12, VBEMP, desktop resolution 800x600 32bpp

Expected result
Pressing F11 switches to fullscreen and should hide the taskbar, e.g. on XPSP3

Observed result
Pressing F11 switches to fullscreen but leaves the taskbar visible

All that it logs upon switching is

fixme:(dll/win32/ole32/compobj.c:3367) no instance created for interface {c43dc798-95d1-4bea-9030-bb99e2983a1a} of class {56fdf344-fd6d-11d0-958a-006097c9a090}, hres is 0x80004002

I suspect the bug to only happen with explorer_new, but not with explorer_old or 2k3sp2 explorer as shell (but did not double-check yet).
I observed the bug for the first time with 0.4.11RC36, but I suspect it is happening much longer already.