LIMITED-SLIP DIFFERENTIAL
A car with an open diff is almost always pretty crap to get off the line; all the power simply spins away through one wheel. Thankfully, a lot of fast Fords already come with an LSD fitted as standard, but incredibly cars like the Mk7 Fiesta ST, Mk2 and Mk3 Focus ST, and even the Mk3 Focus RS all come with an open diff from the factory. And even cars that did come with LSDs, like the S1/S2 RS Turbo and Cosworth range, can all be improved with the latest versions of limited-slip differentials.
The older 80s-style viscous-fluid diffs can be re-built, but for ultimate grip off the line either a plated-type or automatic torque biasing differential is probably better. A limited slip diff improves traction by doing exactly what it says; it limits the slip across the differential. Or, in other words, it forces the available power to both wheels, which in-turn means improved acceleration. And, even if the wheels are spinning, both will have power (or technically, torque) being sent to them which gives more forward momentum than an open diff, which would simply spin all the power away at the wheel with least resistance.
There are different types of differential available, and it depends on what driving characteristics you prefer, but as a general rule FWD cars tend to favour ATBs while reardrive applications are well suited to a plated-type diff.