[WetWired]

As Paul M indicated, it's hard to tell you why it's doing an unspecified undesired behavior. Just looking at the code alone, though:

1. You're making a list of threadids and firstpostids, except that you didn't select firstpostids, so you're making a list of threaids and NULLs, which is probably giving you trouble in the second query. Remove the line adding firstpostids to the list; you're using the list as threadids anyway.
2. Your second query GROUPs BY a post field, but you're joining with the post table on the primary key, so there will never be duplicates; the GROUP BY clause is unneeded and may be causing problems
3. You're joining with the parsed post table, but not using it. This is a waste of mysql time.
4. You limit your results in both queries, which is sure to cause problems when you later increase the first limit and can't figure out why it didn't work.
5. Your current design cuts off a post in the middle of any tags that may be open, leaving them open for the rest of the page
6. When you do the template, you set the string equal to it, instead of concatenating, which means only the last result will be in your string when you're done