So you definitely should count it in net worth as asset value minus liability (mortgage).
For FIRE calculation purposes, here's what I'd do:
1) Calculate what you will net on it, on average, after all expenses (including setting aside money for vacancy and capital expenditures). Keep a healthy reserve fund to deal with those things.
2) Subtract your net rental income from your FIRE budget.
The number that is left is your expenses not covered by your rental income. The amount you need to save in your investment portfolio is 25x (at a 4% SWR) that number.
Your rental value, while calculated in your net worth, is not a part of your investment portfolio, so its value won't affect that number you need for a 4% SWR.
So, as an example, I'll make up some numbers.
Let's say you have a rental that nets you $12,000 annually after all expenses, paying the mortgage, and setting aside for reserves (say it actually nets you 17000 in a year with little expenses and no vacancy, and you set aside 5k, that nets you 12). Let's say your FIRE budget is 35k, and your net worth is 600,000, of which 200,000 is your rental and 400k is invested in the market.
Your budget of 35k - 12k net rental income = 23k you need to cover with withdrawals from a portfolio. 25x 23k = 575000. You only have 400k in your portfolio (your net worth minus the value of the rental), so you need to get another 175k to be FI.
Did that make sense? :)