The line's still very short. 15-20 737-500s are likely to be parked/sold before the end of the year. Those 737s have been planned to go for quite while. I wouldn't be surprised if they are coming up on heavy checks.
There won't be any furloughs unless the cuts go much deeper.

I was furloughed for five years, so I understand your angst. At the moment, I'd say that you'll have no problem clearing probation, even if UAL continues to park aircraft.
Prior to any furloughs, the company will offer liberal leave of absences. Once you see that happen, expect to be furloughed in the next 6 months.

As always, plan ahead. Have multiple furlough plans, including one that does not involve flying. You've got your mainline line number; no need to stay current.
As for a possible furlough, just remember that failure to plan is planning to fail.

Welcome aboard and good luck.