Bug 1203584 was filed on September 10th, 2015 indicates Mozilla has delay the enforced extension signing (unsigned add-ons can not be installed…period) until Firefox 44 coming in late January 2016. This means that unsigned add-ons may still be installed in Firefox 41-43 if you set the xpinstall.signatures.required preference in about:config to FALSE. Not sure as to the reason for this change as the bug simply stated:
We've decided to push turning the pref for signing requirements on out to 43 so need to turn it off in aurora/beta. Forcing the pref on will be pushed out to 44
