[imp] IMP 5.0.0 : unable to write any new message or to reply to previous : cannot open editing window ?

[techie]

 Hello

 after the upgrade to Horde-5 and IMP-5, the New Message or Reply button 
 on existing email does nothing.
 I setup the Horde log to debug level but nothing is logged here.
 Looks like IMP-5 cannot open any new windows for editing.

 Browser is Firefox 16.0.2.
 OS is Ubuntu 12.10 (so both latest versions for Ubuntu).

 Thanks for any clue or help.

 Bye,
 Bruno