Disabling Firebug does resolve my issue. Last version where issue didn't appear was Firefox version 12.0.1
Ubuntu 12.04 64-bit Firefox & global menubar 14.0.1 Firebug 1.10.2
Mozila Firefox for Ubuntu Cannonical 1.0
Disabling Firebug does resolve my issue.
Last version where issue didn't appear was Firefox version 12.0.1
Ubuntu 12.04 64-bit
Firefox & global menubar 14.0.1
Firebug 1.10.2
Mozila Firefox for Ubuntu Cannonical 1.0