hello Vartiovuori, the drag&drop behaviour for tabs has changed in firefox 17 - they now move within the tab strip while being dragged. unfortunately the behaviour that you're now observing seems to be a somewhat intentional by-product of this change, it is unlikey that this will be fixed soon. you'd have to resort to the usual method of right-clicking a tab & select to pin it as app tab.

for reference see bug #787897 (please don't post in bug reports).

I have this problem too. I use this feature everyday, multiple times a day. So for now I downgraded back to Firefox 16, but I am hoping they fix this issue soon!

I Found a working solution: Install the Tab Utilities extension for Firefox and make sure extensions.tabutils.disableTabMoveAnimation is set to true. The animation should now be gone and the lovely arrow back!

Interestingly, if you drag a normal tab out of the window onto your desktop it becomes a tab in a new window. Then you can drag the tab from that window into the app tab area of the original window and it becomes an app tab. This is too tedious to be a work-around, but interesting nonetheless.