How should I cite something that I saw in a famous internet forum (not Stack Exchange)?
In the strictest sense, a citation is exactly that: A citation, not an endorsement. As a consequence, I would find it unlikely that someone would consider your paper any less just because you cite something or someone.
But then you also always have the opportunity to qualify citations. For example, you can say something like the following, which is a pretty strong endorsement of the work you are citing:

In [13], Bayer and Schmidt have previously considered this exact problem and produced an excellent algorithm to solve it. That said, while we know from their work that the algorithm has optimal complexity on average, there are also known cases where it runs for far longer than in the optimal case. This paper addresses this flaw.

Compare this with the following citation:

In preparing this work, we have spent a substantial amount of time on trying to find literature that considers the problem we address here. Very little can be found, with the possible exception of the blog post [42] that presents some ideas -- but few details and no worked out solutions -- of what we do here.

Such a statement makes it clear that you think it is related, but it's not an endorsement because you also make clear that whatever is presented there is not a full solution of the problem. This seems unobjectionable to me: Nobody can be mad at you for citing prior work this way, as you make it clear that whoever goes to that blog post will not find prior art, just ideas.