Well, my mom used to have it, actually.
She uses Thunderbird and Firefox but lost some of her extensions. One of her favorites placed a Thunderbird notifier in one of the Firefox toolbars (similar to my beloved gmail notifier). When you’d get email in any account set up in Thunderbird, it would display a small ‘new message’ graphic. You could then click (possibly right-click) and it would tell you which email address got the email. I believe you could click on it and it would then bring up Thunderbird to view the message.
I can’t find the bloody extension! I’ve found her Forecast Fox (I love that thing) and a few others, but not this one. Anyone know where to get this? I keep stumbling across “update notifier”, but that just tells you if there are updates to extensions you already have.
[sub]Edited to add another sentence for clarification.[/sub]