Does Teferi, Mage of Zhalfir allow you to cast spells at instant speed on your own turn?

Teferi, Mage of Zhalfir has two relevant abilities. First, creature spells you control can be cast as though they had flash, meaning you may cast them any time you could cast an instant — including during your own turn at any point you have priority (CR 116.1, 702.36a).

Second, Teferi's second ability states that your opponents can cast spells only any time they could cast a sorcery. This restriction applies only to opponents, not to you. So on your own turn, you are unaffected by this restriction and can cast your own spells normally — sorceries during your main phase, and now creature spells at instant speed thanks to Teferi's first ability.

It's worth noting that 'casting at instant speed' doesn't create new windows by itself — you still need priority and an open window. But on your own turn, you naturally have priority during your main phase and can also act during combat or your end step, for example (CR 116.3).

Concrete example: On your turn, after attackers are declared (during the combat damage step), you cast Snapcaster Mage from your hand. Normally a creature spell is a sorcery-speed play, but because Teferi gives it flash, you can cast it mid-combat, getting the 2/1 body and its ETB trigger right away.

In summary: Teferi grants your creature spells flash (instant speed) and restricts only your opponents to sorcery speed. You personally gain flexibility, not restriction, on your own turn.