I would like to find my first post overall, not just the first thread I started. Just didn't know if that's possible, but @LiquidFeet sounded like it was.
Okay, now you got me curious. Try this on a desktop/laptop.
1) Use drop down below a username in a post to select Threads Started. That generates a Search.
2) You will see the count of All Posts for the user. For dbostedo, that's a little over 1000.
3) Click on All Posts on the Search results page.
4) Find the last page and go there. For sibhusky, that was over Page 500. For mdf, the last page was Page 171 and I could go to his first post was in 2006.
Normally the Huddler Search isn't worth much. Guess it's good for something.
There you go. Whoo-Hoo to marznc for figuring it out.