As I can tell you from experience, they don't work all of the time, and it is a source of frustration for front-end developers I know who need to work with it.
You're not supposed to use the webkit prefixes. The only reason firefox has them is because some older websites will stop working.
Most of them have equivalent standardized features, or are in a state where you really shouldn't be using them. Don't expect every webkit prefix to work in Firefox.
So yeah, using prefixes to make props work on older browsers is fine. Using only the prefixed version of a property or a property that doesn't have an unprefixed version at all is not.
