Does Firefox just stop there, like it's ignoring a redirect, or does it loop? '''''Redirects''''' Double check that Firefox is not set to block certain redirects: Windows: "3-bar" menu button (or Tools menu) > Options Mac: "3-bar" menu button (or Firefox menu) > Preferences In the left column, click Advanced. On the right side, make sure the "General" mini-tab is selected along the top. If "Warn me when websites try to redirect or reload the page" is ticked, clear that checkbox. '''''Loops''''' Often a cookie problem. Could you try: (1) Make sure cookie settings are liberal for testing purposes. See: [[Websites say cookies are blocked - Unblock them]] (2) Remove existing cookies for microsoftonline.com AND office365.com AND office.com -- assuming you do not have any unsaved work pending on the system. Either: * right-click (on Mac Ctrl+click) a blank area of the page and choose View Page Info > Security > "View Cookies" * (menu bar) Tools > Page Info > Security > "View Cookies" * click the padlock or "i" icon in the address bar, then the ">" button, then More Information, and finally the "View Cookies" button In the dialog that opens, the current site should be pre-filled in the search box at the top of the dialog. Edit that to microsoftonline.com to list that site's cookies, and then remove them individually. Repeat with office.com and office365.com. Then try logging in again and see whether the redirect is successful.

Thank you very much, I've just done all that, and have now succeeded in getting to my mailbox. I'll keep the instructions in case it happens again.