I have some fresh stable manure which is mostly straw and was wondering if it would be suitable for mulching my apple,pear and plum trees
Are you talking about horse manure?
You can use it fresh....but....I would also lay some newspaper or carboard underneath it ..to slow down the 'juices' and allow it to weather and mellow out a bit before it works itself through the layers.
It would be good idea to spread some bonemeal under the cardboard too...it is slow acting fertilizer that will feed the give nutrients for rootgrowth..fresh manure can be more nitrogen rich, so those will 'balance' each other and provide more even supply of food for good growth.
If it is 'steaming' fresh..then you can do it, even if the ground is bit frosted. Cold layer of mulch round tree will just keep the ground colder longer.
If it is cow or other type of manure that is wet/slurry..I would not use it..instead you could try to compost it with something dry mixed in and few months time mulch with that much bulkier 'mixture'.