I studied that in First Order Logic the domain is a set.
In math, a set is defined as a group of elements. But must these elements be numbers ?
Is is possible to have a set S = {'1',"John","11.1"} ?
If yes , is it possible to use the domain S in first order logic ?
Of course it is possible to have a set $\mathbf{S} = \{1,\text{John}, 11.1\}$.
And it is possible to use $\mathbf{S}$ as the domain of the interpretation of some language in first order logic. This means that the (universally or existentially) quantified variables in a formula of such a language range over the element of $\mathbf{S}$. For instance, given a formula $\forall x P(x)$ (for some predicate symbol $P$), if the domain of the interpretation is $\mathbf{S}$, this means that $1$, John and $11.1$ have the property described by $P$.