Which is completely nonsensical. Firefox chiseled away at IE 6's market share with versions 1, 2, and 3. No one is even aware of what version Chrome is on without looking it up.
I can see no reason for this and have not seen a reasonable explanation from anyone on the Mozilla side.
Of course not. Are they good reasons, though? I haven't seen any.
Ask your friend, "Easier for whom?" It's certainly not easier for all those extension developers who would either have to update their extension every 3 months just to indicate compatibility with the new "major" version, or set the maximum version compatibility some 3 major versions into the future, risking breakage down the line.
This would be less of an issue if they fixed their system for determining extension compatibility. Using a "maximum version" and increasing the major version number of Firefox every 3 months is going to [continue to] piss people off.