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It sounds like Firefox's profile has become corrupted somehow. Here is how I would attempt to get around the problem...
1.) Right click your Firefox shortcut and click 'properties' in the context menu.
2.) At the end of the target field add, " -profilemanager".
So the target should look something like...
"C:\Program Files\Mozilla Firefox\firefox.exe" -profilemanager
3.) Click 'Ok' and then open Firefox with that shortcut.
4.) Firefox's profile manager will open up, create a new profile and start Firefox with that new profile selected.
5.) Test the 404 (page/file not found) error pages that have been giving you grief.
If you do not encounter the problem anymore you will probably want to import your bookmarks. Here is an example path that they are buried under...
C:\Documents and Settings\Administrator\Application Data\Mozilla\Firefox\Profiles\Firefox 1.5\bookmarks.html
You will of course have to change your windows username if you don't log in as administrator and the name of your previous profile.
Hope this helps!
- John