I run FTM Mac2 as well, I very seldom run into any program crashes (less often than other software I run, such as iTunes). I will say, though, that I did have a LOT more crashes from FTM before upgrading my Mac in December. My old Mac was a Core 2 Duo with 4GB RAM on OS X Lion; now have an i7 with 16GB ram and Mountain Lion and it's a lot more stable.
If the program is crashing, you may want to run through these steps:http://ftm.custhelp.com/app/answers/detail/a_id/5322
Also it's possible you have some corruption in your tree data file. There are a lot of ways to resolve this, my favorite (i.e. the one that works most reliably for me) is to unlink the tree from Ancestry & redownload it, or if you don't have your tree linked, upload it to ancestry as a private tree then re-download it (you can delete the Ancestry tree when you're done if you don't want it linked)