Restore Session at startup

[Nis Martensen]

Hi,
I am running Debian stable (Jessie) and I have installed the latest
iceweasel (release) according to mozilla.debian.net.
Version: 39.0-1~bpo80+1

When started from the xfce panel, iceweasel always shows the following:

#  Well, this is embarrassing.
#
#  Iceweasel is having trouble recovering your windows and tabs.
#  This is usually caused by a recently opened web page.
#
# You can try:
#
#    Removing one or more tabs that you think may be causing the problem
#    Starting an entirely new browsing session

This only happens when starting from the xfce panel, not when starting
from the menu, and not when started from the command line in a terminal.
And it only started happening after the latest iceweasel upgrade.
Behavior does not change when all add-ons are disabled.

What could be the reason? A difference of execution environment between
the different invocation methods? Comparing /proc/<pid>/environ between
"started via panel" and "started via menu" yields that the only
difference is DESKTOP_STARTUP_ID, which is not set in the former case.

However, it is also not set in the "started from terminal" case where
there is no problem.

I'd appreciate any ideas about how to investigate further.
Thanks, Nis