You're correct in saying that fluoxetine (prozac) can reduce the ability of the chemical that causes MDMA neurotoxicity (whatever it is) to get inside the axon and cause damage. However, in doing so fluoxetine reduces MDMA’s ability to work (as you stated).

People regularly dosing with SSRIs may be safe from neurotoxicity, but they usually don’t feel the normal effects of MDMA either. The idea of using SSRIs to prevent neurotoxicity is something of a catch-22: If you take the SSRI after coming down from the MDMA, it’s probably too late to do a lot of good. On the other hand, taking an SSRI before-hand tends to reduce the desired effects of MDMA, making it more logical to simply take less MDMA in the first place.

All things considered, it’s unlikely that taking an SSRI before or after MDMA is a very useful prevention strategy. Since SSRIs have their own side effects and potential risks, the practice should probably be avoided.

source: http://www.dancesafe.org/drug-information/is-mdma-neurotoxic...