By reset do you mean you are ''resetting the configuration and profiles'' of Firefox, or are you forced to ''restart the browser'' to get Office 365 to work correctly? If just a restart is needed, I would try restarting in Safe Mode, https://support.mozilla.org/en-US/kb/troubleshoot-firefox-issues-using-safe-mode#w_how-to-start-firefox-in-safe-mode, to see if it's an issue with hardware acceleration or Extensions and Themes (Plugin add-ons like Flash are not affected). If this doesn't help, I would try clearing cache and cookies to reset any preferences and logins the site may have, https://support.mozilla.org/en-US/kb/how-clear-firefox-cache?redirectlocale=en-US&redirectslug=How+to+clear+the+cache (you can also search the cookies for all Microsoft and Office related cookies to delete just those so you don't lose active logins on other sites, but clear all of your cache). MS lists Firefox as compatible with Office 365, so things should definitely work.