" ... Add-on signing impacts users and developers to varying degrees. Add-on developers for instance need to submit their add-ons to Mozilla regardless of whether they plan to release it on Mozilla AMO or not.Source: gHacks Tech News
"While it is theoretically possible to skip the submission, it would mean that only Dev and Nightly users can install the add-on as those are the two only channels for which signing is not mandatory. ... "
Details Mozilla Firefox Add-on Signing has started