I don't remember where he originally uploaded each series, but his Jerry Jackson series (a personal favourite) were first uploaded to Newgrounds as a series of troll animations.
He primarily uploads publishes all his material, alongside with Jerry Jackson et al, to his own website[0]. I remember subscribing to it over RSS as a kid and his stuff would always drop there first. Might be some gems to discover if you haven't browsed it before (: Newgrounds, ebaumsworld etc were all reuploads [1].
[1]: At least ebaumsworld was notorious for the bad kinds of piracy. albinoblacksheep, newgrounds, and somethingawful, were the major hubs outside of other individual creator sites like weebls and lemondemon otherwise IIRC. Firth did publish himself on NG, and later on YouTube. But I think he only appeared as himself on YouTube like a decade or so after Salad Fingers first went viral. https://www.newgrounds.com/portal/view/276616https://en.wikipedia.org/wiki/EBaum%27s_World#Copyright_infr...
You are correct - I say this as the website ‘landlord’ (scroll right to the very bottom!)
The fun part was hosting this myself 20-something years ago and seeing traffic spike like mad. Rented cheap servers from Kimsufi or Ikoula and tuning apache as best I could without a real clue what I was doing…
I don't remember where he originally uploaded each series, but his Jerry Jackson series (a personal favourite) were first uploaded to Newgrounds as a series of troll animations.