Firefox hangs in Sandboxie

[Over_the_hill]

I've just installed a registered copy of Sandboxie and have run into a problem with Firefox and Thunderbird.
Both programs run correctly outside Sandboxie but frequently neither appear to run within. Under these circumstances both show on the Control window but do not open. Sometimes, but not always, further clicks on the desk-top icon or the taskbar icon of the program will open it in Sandboxie.
Windows 8.1 64, Avast (free,) Firefox 36.0, Thunderbird 31.5.0 Windows firewall. I've searched through the forums on-line, and tried everything I could find - inactivated Avast in various ways, inactivated my add-ons and plug-ins in firefox...

Any thoughts gratefully received