When I try to install a Firefox add-on from AMO website, a popup window shows a warning message that says :
Install add-ons only from authors whom you trust..
Will this warning message be shown for every Firefox add-on (installed from AMO) regardless of being full reviewed or not? How to not display it for our add-on?
What are the limitations for the add-ons that are granted a preliminary review compared with the full reviewed add-ons?
What is the advantage of signing an add-on?
That's explained in the new distribution agreement and may change over time but at the moment you essentially get a lower search ranking and a scary message designed to inform users that the add-on may be lower quality than the ones that Mozilla have fully reviewed.
The entire add-on signing system will change in the next 1-2 months when the add-on signing requirements come into play so unless you have a very urgent and specific need to sign your add-on in the mean time, you should steer clear of the complexity of the existing add-on signing techniques.