Most people recommend having five years’ cash available somehow in a taxable account or similar to tide you over while you wait for your Roth conversions to season.
Even in a down market your equities will be throwing off some dividends, so that is a start to covering your living expenses.
Worth noting is keeping 5 years of expenses in cash is 20% of your portfolio if you FIREd with 25 times annual expenses. If you're not including that in your bond/fixed income asset allocation it will have a huge drag on the portfolio's overall performance.
Mathematically, the smarter move is to simply make one's normal, periodic withdrawal. In a recession, the market isn't at the bottom long. If one makes quarterly withdrawals you sell a little bit on the way down and back up. Maybe you don't even sell at the bottom at all because it happens in between quarters. Also, you're only selling enough for the quarter so it doesn't feel like a huge hit psychologically.
If one needs the cash to feel secure maybe it's a worthwhile sacrifice, but there is definitely a performance hit to doing so.