Go to the box in your browser that shows the URL. It will read something like:
http://www.freerepublic.com/focus/fr/548277/posts?q=1&&page=581
Delete the 581 at the end and replace it with:
http://www.freerepublic.com/focus/fr/548277/posts?page=586#586
This will take you to that exact reply #. If you have your preferences set for 20 replies per page, you could also write it as:
http://www.freerepublic.com/focus/fr/548277/posts?page=581
http://www.freerepublic.com/focus/fr/548277/posts?page=5221
That will take you to the top of the page that has that reply # and the next 20 replies. (Obviously you would enter numbers in multiples of 20. If you have your preferences set for 50, then use multiples of 50).