If you are trans, nonbinary, or any sexuality/gender that that user was being bigoted against— take a deep breath.
Sit in yourself and find your center. Search for it in your body, were the feeling of truth and love is strongest. Mine is usually in my chest or throat.
Keep breathing and feeling. Acknowledge how much the post made you feel. The anger, the hurt, the disappointment, the disgust, they are important and should be felt. You have every right to feel them. Let them flow through you and let them go if they are causing you harm. Say them out loud, pray, write them down, paint them out.
If you are scared or feel overwhelmed, I would encourage you to get that out in healthy ways as well. For me that means reaching out to others and being comforted by them. Find something that works for you.
Repeat what you know to be true—that the TERF was wrong, that they are bigoted, that there is a community that loves and accepts you.
There will always be people that love you and will stand with you. Your back is never exposed.
If you want to engage with the TERF, then make sure you are in an emotional place that is healthy. If it is going to hurt or upset you, I’d recommend that you let someone else with more energy/spoons than you handle it.