" ... 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.
"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. ... "

Source: gHacks Tech News
Details  Mozilla Firefox Add-on Signing has started