Mozilla Firefox 28 lacks transparency at the scroll-wheel-overlay, it should periodically poll for background color

0.4.8-RC-24-g795c7f7 gcc dbg

Firefox 28 from rapps.
VBox 4.3.40 without any additions, VBEMP graphics driver.

-Open a webpage (with preferrably dark background)
-Click the middle-mouse.button once.
-A scrollwheel overlay symbol is shown.

Observed result
It lacks the transparency at its edges. ff28.png
The log shows nothing, when I invoke that scrollhandler.
But I attached the logfile anyway:0.4.8-RC-24-g795c7f7__FireFox28.log
0.4.10-RC-30-g51a70dc__aLightBulbOverMyHead.webm

Expected result
xpsp3_FF28__ok.webm
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SideNote: Firefox 3.6.28 is affected at well.