This article outlines the ethernet interface speed specifications on MX series routers.
M series and T series routers that have gigabit ethernet interfaces are not capable of supporting a speed of 10M or 100M. They always default to 1000M (1gig)
On MX series routers however, the speed can be set to 10/100/1000 by using the command
set interfaces <int> speed <10/100/1000>
The above only works for copper based (UTP) DPCs/FPCs on the MX series routers. Other DPCs that are SFP based will not support anyother speed than 1000M or 10gig
Examples of UTP based DPCs:
Examples of SFP based DPCs: