If I have a Mageseeker Warden at a battlefield, and then play Firebrand Raider does Mageseeker Warden's "can't" effect stop the enemy units from getting ready when Firebrand Raider enters, and also stop the enemies units from entering ready

Partly — the "ready all enemy units" trigger gets shut off, but the "enter ready" part isn't affected at all.

Mageseeker Warden is a static prohibition: spells and abilities can't ready units that are enemy to its controller. Since you control both Warden and Firebrand Raider, the opponent's units are enemy units to you, so Firebrand Raider's "When you play me, ready all enemy units" ability does nothing while Warden is at a battlefield — it can't flip exhausted enemy units back to ready. But "Other units enter ready" is not a readying action: units arrive already ready and are never seen as exhausted, so Warden can't touch it — same reasoning as Accelerate, Sun Disc, and "I enter ready" units (FAQ #2897, #4300). So your opponent's fresh units still enter ready, and Firebrand Raider's own "I enter ready" applies normally.

Two follow-on notes: Firebrand Raider's "Units don't ready during the Awaken Phase" is a restriction, not an ability that readies anything, so Warden doesn't interfere with it — nobody readies in the Awaken Phase while Firebrand Raider is in play. And Warden's other clause only limits where opponents can play units, so it doesn't restrict you playing Firebrand Raider to a battlefield.