Answer
For the Junos OS Release 10.0 and earlier, the ifIndex is persistent when reboots occur if the Junos OS version remains the same, meaning the values assigned to the interfaces in the ifIndex do not change. When there is a software upgrade, the device tries to keep the ifIndex persistent on a best effort basis.
For the Junos OS Release 10.1 and later, the ifIndex is persistent on all platforms, except for the EX4200 virtual chassis because it can have over 500 interfaces.
For a list of supported standard and enterprise-specific MIBs, click SNMP MIBs and Traps Reference.