Your first post was
this
Your first thread was
this (quite cringey first thread if I do say so myself)
You have to have Forum vip/tp to forums search.
how you do it is by going to the statistics bar when on someones profile.
There will be to links that say
Find all posts by *user*
Find all threads by *user*
You then click to the end of the pages to find the ones made longest back.
Hope I helped <3