Has this been done? Create a mix breed?
You can breed a dog with a wolf, but not a cat. Does this same type of cross breeding restriction work the same with plants?
Can all trees cross/breed?
Thanks!!
I have never herd of attempts to cross a Cedrus (true Cedar) with Abies (Fir) though this may be possible seeing they both belong to the same family (Pinaceae) and there are other such family groups whose disparate individuals were successfuly hybridized, though not many. Most hybridization is done within a genus: Dogs and wolves are both Canis for example. Likewise, most plants and trees can not be crossed between species (above the genus level).
However, your question gets even more convoluted due to a common name …
The common term “Cedar” is a pretty confusing one. As with many common names of flora, it is used to describe several different species: There’s the Eastern Red Cedar which is actually a Juniper, Juniperus virginiana. Then you have the Western Red Cedar and Eastern White Cedar which are both Arborvitae (Thuja plicata and T. occidentalis respectively). Furthermore, there are guys like the Port Orford Cedar (Chamaecyparis lawsoniana ), the Atlantic Whitecedar (C. thyoides) and the Alaska Cedar (C. notkatensis).
As for the Fir, most are just that, Abies, except for the Douglas fir which is Pseudotsuga menziesii.
Confused yet? 
hang on…
The closest crossing relating to your question that I know of is The Leyland Cypress. The Leyland Cypress (x Cupressocyparis leylandii) was made by crossing the Monterey Cypress (Cupressus macrocarpa) and the Alaska Cedar (Chamaecyparis notkatensis) - Two separate, though relatively closely related, species making a third.