You're right, 1.12.6 doesn't seem to work in Nightly. @Honza, I see the following error in the error console:
WARN addons.xpi: Exception running bootstrap method startup on
fir...@software.joehewitt.com: [Exception... "Component returned failure code: 0x80520012 (NS_ERROR_FILE_NOT_FOUND) [nsIXPCComponents_Utils.import]" nsresult: "0x80520012 (NS_ERROR_FILE_NOT_FOUND)" location: "JS frame :: resource://firebug/loader.js :: FirebugLoader.startup :: line 74" data: no]
I assume that should be fixed with
https://github.com/firebug/firebug/commit/10af7f569a, right? So we also need to backport that to 1.12.
Sebastian