DICE has allready stated that they can't release mod tools for number of reasons:
1. because of licensing issues (DICE does not own all of the tools needed to make changes to BF3 and it would be too expensive to make them avaliable for mod support). For one, tt was faster and cheaper for DICE to license the user of new animation tool sets from another company instead of making their own.
2. The frostbite engine is not designed to be easy to mod. Instead, It was designed to be easy for different game devleopers to be working on different aspects of the game as the same time across a local network.
The best we can hope for in modifying BF3 is a sandbox type level editor - where you can adjust spawns, flag locations, type of vehicles, ticket count, etc. But as far as importing new vehicles and weapons, that is not going to happen with BF3.
If there are mods for the game, they will be very small, and most likely created for cheating, such as aimbots and unlocks.
Even if DICE releases mod friendly version of the Frostbite engine in BF Bad Company 3, BF4 or some other game, it still may not be suitable if it does not support the large space and land maps currently in First Strike. Understand that BF3 uses a completely new engine from BF2142 (Frostbite instead of Refractor) that may not support the scope that the First Strike team wants to develop.
If First Strike lives beyond BF2142 it will have to be with a mod friendly game engine, not BF3.