Do posts on writing community websites count as granting first rights? About six years ago I had the first draft of a story I'm currently trying to gain representation for (the current version has a doubled word-count and completely different POV over the draft I posted but the overall gist of the story is the same) available online. Do first drafts count in these types of situations? Is my paranoia justifiable or am I sliding into nitwit territory?
There is no such thing as "first rights" for books. I think you mean first serial rights.
First serial rights are rights to print excerpts of a book before publication. The question isn't whether you've given them away..you have. The question is whether you can sell them again. You have to read the submission guidelines of the magazine or contest or whatever place you want to sell/submit them to see if they will take "gently used words".
Book publishers don't buy first serial rights. They buy what's called world rights, or North American rights, or some such geographical description of the right to publish your work in BOOK FORM.
If you're well known enough, publishers will buy all sorts of things that have been published previously (Maureen Dowd of the New York Times comes to mind instantly, as does Dan Savage of The Stranger).
You may have seen that publishers aren't interested in things that have been published on the web but it's because they think you've tapped the market sufficiently for the work, not that you've sold the rights they'd need to have to publish.
You may have seen agents who say "we don't take on anything that's already been self published". It's not cause they can't sell those rights, they could. They just don't want to bother with it cause generally that's not a very profitable market.
There's a distinction between "don't want to bother" and "not available to be sold".