Yeah, that one is apparently significant enough to be a 'sticky' at the Firefox Bugs forum:
Major Bug!!!! "The file / cannot be found" - MozillaZine Forums, and in Bugzilla at
Bug 123662 - "File / cannot be found. Please check the location and try again" when loading page.
On the other hand, I Fark and I've never seen this bug. Someone recommends changing browser.cache.check_doc_frequency to 1, which I use as well.
(coming back to the thread)
The setting I told you above changes the 'check for new version of the page' to 'every visit'. Probably not ideal for someone on dialup, but I consider it necessary for a highspeed user. Sometimes, a site will have a long 'expiration' setting, and be updated in the meantime. I always want the latest version of a page right now, and I'd rather take the time/access hit.