Thank you for your post.
I am unable to replicate the problem. Here is what I have done...
1. I hosted a database file with a Web Viewer on FileMaker Server 11v2 running on Windows 2003 Server.
2. On a Windows XP machine, I launched Firefox 3.5.13, accessed the file via Instant Web Publishing, right-clicked the graphic in the Web Viewer, selected "View Image", and then after viewing the image in a new page, I pressed the back button and returned to the previous page.
3. On a Mac OS X 10.5.8 machine, I launched Firefox 220.127.116.11, and repeated the same steps above. No errors encountered when returning back to the preceding page.
Please let me know what I'm doing differently than you, and I'll try to replicate the problem here.