Member Rara Avis
It's not a browser issue, but rather a security issue. Internet Explorer does the same thing and there are a couple of threads in Bug Swatter about this known problem. I think Balladeer was the first to raise it, shortly after IE 6.0 came out.
When you post something, whether new reply or comment, the system prints a page to let you know you succeeded. Feedback, so to speak. That's probably not where you want to be stuck, however, so that same page includes a meta-refresh set to something like ten seconds, which should automatically redirect the browser back to the thread or forum. It takes you where you probably wanted to go. Unfortunately, redirects can also be used to take you where you DON'T want to go, so many modern browsers now include settings to completely disable those redirects. You need to track down that setting, looking for something referencing refresh or redirect, and change it.