Comment 3 for bug 1375819

I had the same problem when installing fonts from .ttf files. Either the system froze completely and I had to reboot manually or just the font viewer window froze, in that case I could right click it in the dock and close it.
However, I found that a bit of patience with the program helps to prevent the freezing. When I wait about 10 seconds after I opened the font in the viewer and before I click install and do the same after it said "installed" and before I close the font viewer, no crash occured.