Please consider turning on send info to Mozilla from with in Firefox before posting a question. Or do not block info. This helps us help you. I have no info from your system that would help see things that cause issues. Hi, no info on your Firefox version #, no info on the add-ons you are using or video card drivers that maybe corrupted. Have you gone to the maker of Sharepoint calendar to seek help : https://sharepoint.stackexchange.com/search?q=firefox+sharepoint+calendar SAFE MODE *https://support.mozilla.org/kb/Safe+Mode *https://support.mozilla.org/kb/Troubleshooting+extensions+and+themes In Firefox Safe mode these changes are effective: *all extensions are disabled (about:addons) *default theme is used (no persona) *userChrome.css and userContent.css are ignored (chrome folder) *default toolbar layout is used (file: localstore-safe.rdf) *Javascript JIT compilers are disabled (prefs: javascript.options.*jit) *hardware acceleration is disabled (Options > Advanced > General) *plugins are not affected *preferences are not affected TEST'''''''''is issue still there ?