Lol well this one certainly is an interesting one. I systematically stripped out all HTML within photo.php until eventually I'm left with just the code for the Ajax Tabs Content script. Still same weird text shows up when fetching one of the external files. There is something strange with the encoding of those external files (ie: events.php), but I can't tell what that is. As soon as I substitute events.php with a normal page I created from scratch, the content inside that page is fetched with no strange characters in Firefox.
Best I can say now is try changing the URLs to these external files within your HTML from something like:
Perhaps the first reference triggers some sort of strange server response creating the weird encoding.