This should be in the General sub-forum, if seeking help here, as far as I'm aware this is a firefox issue.
The first thing I would suggest is updating FF as it is now on 44.0.2.
Second, what FF theme are you using ?
Have you tried using the default theme and see if you still have the same issue.
I have many pinned tabs and they are still present and they are displaying the correct icons.
I also use Tab Mix Plus to manage Tabs, so I don't know if this also manages the icons.