https://support.mozilla.org/en-US/kb/update-firefox-latest-version '''46.0.1''' Are these users using the same computer or separate systems? Many site issues can be caused by corrupt cookies or cache. * Clear the Cache and * Remove Cookies<br> '''''Warning ! ! '' This will log you out of sites you're logged in to.''' Type '''about:preferences'''<Enter> in the address bar. * '''Cookies;''' Select '''Privacy.''' Under '''History,''' select Firefox will '''Use Custom Settings.''' Press the button on the right side called '''Show Cookies.''' Use the search bar to look for the site. Note; There may be more than one entry. Remove '''All''' of them. * '''Cache;''' Select '''Advanced > Network.''' Across from '''Cached Web Content,''' Press '''Clear Now.''' If there is still a problem, '''[https://support.mozilla.org/en-US/kb/troubleshoot-firefox-issues-using-safe-mode Start Firefox in Safe Mode]''' {web link} While you are in safe mode; Type '''about:preferences#advanced'''<Enter> in the address bar. Under '''Advanced,''' Select '''General.''' Look for and turn off '''Use Hardware Acceleration'''. Poke around safe websites. Are there any problems? Then restart.

See also: *https://support.mozilla.org/kb/troubleshoot-SEC_ERROR_UNKNOWN_ISSUER