This is usually caused by an add on misbehaving. If you have any addons try restarting with add ons disabled and see what happens. You can do this from the Help menu.

Thanks for your reply. Unfortunately, starting with Add-ons disabled does not solve the problem.

Could you please tell the forum what font your are using. Is it possible you are using a font that does not have separate bold and not bold entries ?

Tools > Options > Display > Formatting Tab Fonts & Colours: Default font: ?

Post this info: In Thunderbird Help > Troubleshootin Information do not select / uncheck 'include account name' click on 'copy text to clipboard' paste info into this question.

Using Calibri. I have never changed the font. Had no idea how to.

TRy this : reindex the Inbox folder, move all emails and then compact folder. These are the instructions:

• right click on Inbox folder and select 'Properties'
• click on 'Repair Folder' button
• click on OK

• Refresh the folder pane. Either select another folder and then reselect the Inbox or collapse all folder trees to refresh the folder Pane.

• Then move all good emails into suitable folders.
• Delete any emails you do not want.
• When Inbox is empty; Right click on Inbox and select: Compact

Wait for a new mail to see if it is displayed in bold.