Autodesk FBX viewer is dead in Yosemite and isn’t going to be fixed until later this month with the release of a new FBX suite. Therefore I have had to look for an alternative and have tried many viewers some nice some not the problem is few open FBX by association if at all.
There is one program that does open them and reads them quickly and that is Marmoset Toolbag 2 I had just been using Toolbag to open files and then thought, hell why not just associate the files to the program so they open there by default.
It opens fast and is a great viewer. It will open multiple files into the same view which is great if you want to compare two models a High Poly and a Low Poly for alignment for example. The more I think about it the more I realise this is a far better solution than the poor little FBX viewer from Autodesk, so unloved it has been left dead since Yosemite was launched months ago.
Toolbag is a light fast program and on my machine loads easily as fast as Autodesk’s program did, but has a lot pf extra features built in like object selection hiding manipulation and of course you can add some cool textures if you want, it is a pretty hot renderer too.