See https://support.mozilla.org/en-US/questions/1347591

After reading of possible connection of non-bolding problem to BitDefender (which I use), I contacted BD and explained problem. They gave me a series of steps including deleting SuperAntiSpyware program which I also had on my machine and going through steps to re-install BD. Unfortunately, after all that the problem remained unchanged. I used the View menu in TBird to deselect the BitDefender AntiSpam Toolbar but still no difference. I then tried the suggestion to switch off the AntiSpam protection feature in BD and found the problem had disappeared and all new messages going into the INBOX were once again properly bolded. I informed BD support of the results above and they responded as follows: ------------- From BD Support Center. Thank you for your patience throughout the entire process and please accept my apologies for the frustration that we might have caused. I have looked into the case and it seems that the situation is caused by an issue that has already been acknowledged by our development team. We are currently working on a fix which will be done by means of automatic update and won't require any additional action on your end. I will get back to you with an update regarding the fix as soon as we have one. Thank you once again and have a nice day ahead! Best regards, Marius N. Customer Support Engineer --------------- It seems this should be corrected soon by BD.