Pinned Tabs not visible, using groups Firefox 37/38

[Olfried2]

I have several tab-groups. I pinned some tabs to the vertical-tab space.
In the "first-made" group, the tabs are correctly displayed but in the "second and further made" group the pinned tabs dissapear. (actual I have six groups).

This beahviour started with firefox 37. OS is windows 7 prof.