Same problem here: FF 24 Linux x64, Firebug 1.12.*, 1.13
Every AJAX request generates 2 warnings and clicking on the Firebug console messages opens up the message empty and generates the error message.
Additional info maybe, that at the same time the issue appeared the Firebug console messages came from FirePHP lost their formatting - they appear only monospaced fonts, but white background and with no icons (maybe because of the "Error during template postRender"?)
I experience this issue since upgrading to 1.12. I had to revert back to
Firebug 1.11.4 since this is the last version working with the Companion
well (although since Firefox 24(?) it generates warnings at ajaxified FirePHP messages, but they
are far less annoying than not seeing the details of the debug messages
:) )