I tried Opera when I saw IM enevelopes while on Fx (just immediately switched to Opera) and I left the envelope there and wandered around the site and the envelope (two different folks with IMs they sent) never disappeared. I finally opened one IM on Opera and the other one was still there (the notification) for me to open.
But tonight, different story. I had three IMs waiting for me this afternoon. Two from one person. I read the one that was from other person and it was too long to read until I opened the IM history page for that person. This was on Fx. I left that tab open when I left my home a little while later. I didn't reply to the IM. I still had two from another person waiting to be read. So, I just got home and brought up that tab and no envelope at the top of the page. This has happened many times recently. I don't think it was always this way (but my memory is not what it used to be so ....) anyhow, there is no indication that there are two messages I have not yet read. Is that because I left the tab open to the history of the other person who sent an IM and I read that IM ON THE HISTORY PAGE NOT FROM THE LITTLE BOX AT THE TOP (because it was too long)?
It seems that every time I get several IMs waiting for me, and these are chatty folks both of them (I am chatty too), frequently I have to go to the person's history page to read the entire IM. When I do that the IMs from anyone else still waiting to be read SOMETIMES disappear from the top but not always. This is usually on Fx because that is my default browser. Then this afternoon, they were still waiting at the top of the page, (unread with the envelope), when I left the house. I got home and the notification was gone. It's not on Opera either yet I have NOT read the two IMs from the other member.