I concede that you can jury-rig the inputs to include a fixed mortgage or part-time work in Firecalc (unless you have more than 3 pension/off-chart spending inputs), what I am suggesting is that a fixed mortgage or part-time work are common enough that there should be a more intuitive way of inputting them... like the section inputting SS or pensions.
You can easily model a mortgage by an non-inflation adjusted off-chart spending (in the pension section of the Other Income/Spending tab) item offset by a same sized pension starting in the year your mortgage ends but it would be much more intuitive if there was a section that allowed for input of your current annual mortgage payments and the year your mortgage ends.
I like Firecalc and am familiar enough with it to jury rig things but it isn't intuitive a lot of others... I'd love to see Firecalc get closer to the intuitiveness and ease-of-use of Quicken Lifetime Planner.