Why are e-mails (subject line) and ("to" line) still staying in bold print after they have been read?

After I read an e-mail, and after I hit the back space arrow, to look at my list of e-mails, the previously read e-mail still shows up in bold type. Usually when an e-mail is read, and you then look at your list of e-mails, the e-mails that have been read show up in light grey type so you can distinguish these e-mails from the ones that have not been read. Why is this happening? It is very difficult to remember which ones I have read when I am looking at over 100 unread e-mails. == This happened == Every time Firefox opened == yesterday