I think you are blaming Firefox 3 when when the actual problem is the
way one of your posts is coded. If I view in Firefox 2 and click
Posting Lama to view the second three, the sidebar strays off to the
right.

Since I don't know what you will see while editing, here's my best
shot:

-Edit "PDF_Password_Remover..."
-Switch to Edit Html.
-Find </center> near the end.
-Delete it from that location.
-Insert it BEFORE <div class="fullpost">, so you are left with:
 </center><div class="fullpost">
-Publish and view, looking at the second page.
-If the sidebar is up, change your number of posts per page to
whatever you like and recheck the page.

The post editor starts in the last Published mode, so, the next time
you create or edit a post it will be in Edit Html. Switch back to
Compose the next time if that's the way you normally work.