@clarkinks has a thread every year where he rates fireblight-resistant pears. He rates Warren an 8/10:
So far as I know, the rootstock does not affect the fireblight resistance of the grafted tree. Rootstock resistance still matters, though, because a root sucker can grow out of the rootstock. If the rootstock is not a resistant variety, the sucker can get infected by fireblight, either before you prune it, or through the pruning wound itself if you are unlucky.
If fireblight travels down an infected sucker and reaches the roots, the tree is done for.