Hi, really sorry but this is a known bug which hopefully will be fixed in an update. https://bugzilla.mozilla.org/show_bug.cgi?id=1433591 Currently known workarounds: Switch from the classic layout to the new beta layout (there's a switch above the Inbox to "Try the beta") Use the Reading Pane on the right side of the Inbox to avoid viewing the messages overlaid on the inbox (requires a wide display, reduces the number of messages shown in the list) When the problem appears, use zoom, and to navigate between messages use the up and down arrows at the top of the current messages instead of closing and returning to the list This is an effective workaround but the implications of using it are unknown: (1) In a new tab, type or paste about:config in the address bar and press Enter/Return. Click the button promising to be careful or accepting the risk. (2) In the search box above the list, type or paste stylo and pause while the list is filtered (3) Double-click the '''layout.css.stylo-blocklist.enabled''' preference to switch the value from false to true (4) Double-click the '''layout.css.stylo-blocklist.blocked_domains''' preference and set the value to live.com and click OK. (In the unlikely event that you already had one or more sites listed add ,live.com to the list.) Then exit/quit Firefox and start it up again to make the change effective. ''If your question is resolved by this or another answer, please take a minute to let us know. Thank you!''