If you want to jump to the first or last post in a thread, there’s a quick way - tap or click on the date/time above or below the slider. Here’s a pic on mobile:
From a list of threads you can go to the top or bottom depending on where you click:
Desktop Only
On desktop you can also use the Home key to move to the first post in a thread.
Use the End key to move to the last post in a thread.