How to run a program with SCHED_RR policy from command line?

Use the chrt command with chrt --rr <priority between 1-99> <command>

Example:

chrt --rr 99 ls

Note that setting SCHED_RR require root permissions, so you either have to be root or run it with sudo.

You can also use chrt to give a running process realtime priority:

chrt -p --rr <priority between 1-99> <pid>

The same commands applies for other scheduling classes as well, albeit with a different parameter instead of -rr:

Scheduling policies:
  -b | --batch         set policy to SCHED_BATCH
  -f | --fifo          set policy to SCHED_FIFO
  -i | --idle          set policy to SCHED_IDLE
  -o | --other         set policy to SCHED_OTHER
  -r | --rr            set policy to SCHED_RR (default)

Edit:

In the Firefox case, it must be spesific to Firefox. In a multithreaded application I wrote myself, all the threads keep the RR class. As seen in your output, two threads have class RR, so it's not only the parent thread either.

Edit 2:

Try starting the process with chrt instead of rescheduling an existing pid. It appears that if you reschedule, only the first thread gets RR class. However, if you start it with chrt, every thread gets it.