In this scenario, Yixlid Jailer's ability prevails, correct? As it is a 'negative buff' which precedes other abilities? Or can someoone explain this interaction better please?
Actually, in this scenario, if Yixlid Jailer was already on the field when Snapcaster Mage's "enter the battlefield" ability resolves, the card does have flashback until end of turn. This is mainly because, generally, when continuous effects that modify the same kind of thing (type, color, abilities, etc.), they are applied in timestamp order. Yixlid Jailer removes abilities, and Snapcaster Mage adds abilites, so when we consider how the effects are applied, first Yixlid Jailer removes all abilities of all cards in graveyards (because it entered the battlefield first), then Snapcaster Mage adds flashback to one card. So the net result is that that card has flashback.
If instead Snapcaster Mage's ability resolved first, it would give the card flashback and then Yixlid Jailer would remove the abilities, so the card would not end up with flashback. However, in most cases you can either cast the targeted spell in response to the Jailer or simply cast it before they have the opportunity to cast the Jailer (in the case where you cast Snapcaster on an empty stack and you are the active player).
The comprehensive rules say:
One important thing to note is that Yixlid Jailer doesn't say "Cards in graveyards lose all abilities and can't have or gain abilities," so it doesn't stop the card from gaining flashback.