Here is my contribution to this interesting discussion. Introduce $T_{\le n}(z)$ the OGF of full binary trees of height at most $n$ by the number of nodes. Now a tree of height at most $n$ either has height at most $n-1$ or height exactly $n.$ The latter tree has a subtree of height $n-1$ on the left or the right of the root node or the root has two children of height $n-1.$ This gives
$$T_{\le n} = T_{\le n-1} + 2z (T_{\le n-1}-T_{\le n-2})T_{\le n-2} + z(T_{\le n-1}-T_{\le n-2})^2$$
where $T_{\le 0} = 1$ and $T_{\le 1} = z+1.$
Observe that $$T_{=n} = T_{\le n} - T_{\le n-1}.$$
Some algebra produces the simplified form
$$T_{\le n} = T_{\le n-1} + z T_{\le n-1}^2 - z T_{\le n-2}^2.$$
This produces e.g. the following generating function for trees of height at most four by the number of nodes:
$${z}^{15}+8\,{z}^{14}+28\,{z}^{13}+60\,{z}^{12}+94\,{z}^{11}
+116\,{z}^{10}\\+114\,{z}^{9}+94\,{z}^{8}+69\,{z}^{7}
+44\,{z}^{6}+26\,{z}^{5}+14\,{z}^{4}+5\,{z}^{3}+2\,{z}^{2}+z+1.$$
For the count compute the sequence $T_{\le n}(1)$ which yields
$$1, 2, 5, 26, 677, 458330, 210066388901, 44127887745906175987802,\\
1947270476915296449559703445493848930452791205,\ldots$$
This is OEIS A003095 and has closed form recurrence
$$t_n = t_{n-1} + t_{n-1}^2-t_{n-2}^2$$
with $t_0=1$ and $t_1=2.$
The number of trees of height exactly $n$ is given by
$$t_n-t_{n-1}$$ which gives the sequence
$$1, 3, 21, 651, 457653, 210065930571, 44127887745696109598901,\\
1947270476915296449559659317606103024276803403,\ldots$$
which is OEIS A001699.
Remark. Reviewing this post several years later we see that we have not employed the definition of a full binary tree as shown e.g. at Wikipedia because we admit trees where one of the children is a leaf. But the OEIS says we have the right values, how to explain this. We can count full binary trees by not admitting leaves (trees of size zero) so that $T_{\le 0}(z)=0$ and $T_{\le 1}(z) = z.$ This gives starting values for the recurrence as $0$ and $1$ with the next value being $2.$ However $1$ and $2$ are the starting values for ordinary binary trees, which explains the matching values (shift sequence and prepend a zero term). Hence the above calculation goes through. (Note: we take height zero to be the height of a leaf and height one the height of a singleton.)