The read messages in the inbox are not differentiated from the unread messages for any mail server like ymail.com,gmail.com etc. Please check on the problem.
Whenever I open yahoo mail and gmail inbox, my messages that are already read and those that are unread are not differentiated. Please address at the grievance and revert ASAP.
Please check if this happens in a new profile. If it's okay, you can later copy the personal data from the old profile. Firefox stores your personal data and settings in another location separate from its files/folder. A new profile would have the default Firefox settings (Tools (Alt + T) > Options and about:config ) and usually would also be empty of any Extensions and themes (Appearance) in Tools > Add-ons) and their settings. Also, a new profile would have no previous stored history (Tools > Clear Recent History).