I had the very similar bug in RSS feeds ([bold blue highlighting in RSS](https://bugzilla.mozilla.org/show_bug.cgi?id=582960)). The bug is fixed now, but periodically I meet the topic bug as well. Top folder (account folder) becames bold blue (with meaning of fresh arriving messages, under presence of other unread messages), while there is no bold blue subfolders, neither new messages in the folder/subfolders. I figured out that the most probable reason is ... spam filtering! The new message go into Inbox folder and/or filtered to a subfolder (the folders become bold blue), then Thunderbird spam-filter is applied, moving the message to Spam folder (which is a special child folder of the top account folder). The spam messages are automatically are marked as read, OR the spam folder never get "bold blue" status (that's OK - nobody is interested in new spam arrival). However the top folder(s) still remain "bold blue". The Thunderbird is on 'release' update channel and is updating automatically. The bug is rare indeed, but somewhat frustating.
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I had the very similar bug 582960 in RSS feeds ([bold blue highlighting in RSS](https://bugzilla.mozilla.org/show_bug.cgi?id=582960)). The bug is fixed now (WORKSFORME), but periodically I meet the topic bug as well. Top folder (account folder) becames bold blue (with meaning of fresh arriving messages, under presence of other unread messages), while there is no bold blue subfolders, neither new messages in the folder/subfolders. I figured out that the most probable reason is ... spam filtering! The new message go into Inbox folder and/or filtered to a subfolder (the folders become bold blue), then Thunderbird spam-filter is applied, moving the message to Spam folder (which is a special child folder of the top account folder). The spam messages are automatically are marked as read, OR the spam folder never get "bold blue" status (that's OK - nobody is interested in new spam arrival). However the top folder(s) still remain "bold blue". The Thunderbird is on 'release' update channel and is updating automatically. The bug is rare indeed, but somewhat frustating.