The competition between the big browsers today has increased and it's very important to have a lead advantage.
The flaws will be corrected with Azure and is meant to give Mozilla a lead.
IonMonkey will also contain a new interface for developers who wants to review the code.
Both IonMonkey and Azure will be optimized to minimize interference with the memoryhandling.
The changes probably wont come for FX5 but will be released for later versions. Next big update is scheduled to June 21.