Are you sure this is all one page? It's possible that only a subframe (which happened to be 100% width and height) failed to load, and the warning is from another subframe or the main page.
I guess I'm not 100% sure :/
Next time this happens, right-click and see if you get menu items related to frames.
This is a mass search for Firefox General bugs filed against version 3.0 that are UNCO and have not been changed for 200 days. Reporter, please update to Firefox 3.6.10 or alter. Firefox 3.0 is no longer supported and is no longer receiving updates. After you update, please create a fresh profile, http://support.mozilla.com/kb/managing+profiles, and test to see if your bug still exists. If you still the bug, then please post a comment with the version you tested against, and the problem. If the issue is no longer there, please set the RESOLUTION to RESOLVED, WORKSFORME.
No reply from reporter, INCOMPLETE. Please retest with Firefox 3.6.12 or later and a new profile (http://support.mozilla.com/kb/Managing+profiles). If you continue to see this issue with the newest firefox and a new profile, then please comment on this bug.