Is it permissible to use the phrases "O companions of Paradise" or "O companions of Firdaus" for addressing people, even though we do not know the unseen and our destinies?

The ruling on this statement depends on the intention of its speaker:

If the intention is that this companionship helps him in obedience and staying away from sins, then there is no harm in this intention, as long as it does not involve falsehood or contradict reality.

However, if the intention is that this companionship consists of the people of Paradise, then this is forbidden because it is considered an arrogation to Allah and speaking without knowledge. Only Allah knows the people of Paradise, and the Prophet, peace and blessings be upon him, forbade self-praise or definitively declaring anyone to be among the people of Paradise or Hellfire.

Therefore, such unrestricted statements and designations should be prohibited due to what may result from them, such as arrogating to Allah, lying, or undeservedly praising oneself.

And Allah knows best.