More browser related long shots:
Browser Plugins or Extension? Any odd ones that can be removed?
Once your cache was cleared and browser restarted, did you type in IIS URL from the address bar and have no auto fill etc, sign of a good cache clean.
Links from shortcuts/menu items not quite correct?
Hacked! ? Hopefully not.
HTTPS issues, Site Certificate issues? Popups may be disabled to show these?
Update to latest version of browser, (that one rarely works for me lol).
There is also something I read recently too about a reset option in the browser :
https://support.mozilla.org/en-US/kb...s-fix-problems
Cheers.