Thunderbird and Firefox do not share add-ons. For the last few version, Firefox add-ons will not run in Thunderbird and vise versa.

There is the ability to "inject" add-ons at run time. Mostly used unfortunately by anti virus companies that think they are exempt from good practice. So I would suggest you look in the add/remove programs for something that might be putting it in.

I am not however aware of a method to "hide" an add-on, so if it is installed it will appear in the add-ons manager.

Thunderbird and Firefox both report browser plugins (not add-ons) these are registered in the registry and both programs will load them if they are installed. They are shown in the plugins section on the add-on manager.

I appreciate your quick response. My apologies for the long reply. I hope there's something for everyone in this.

It seemed very odd that it could happen. As it turns out, it was a different issue.

BTW - and it is an extension, not an add-on. My apologies for the confusion.

The extension was installed at the same time as going through the application options. One of the options enabled a feature that it called "Show button in browser title bar". This is related to capturing URLs in a browser. The documentation was clear about only web browsers. It didn't click that an extension should do the same kind of thing.

The extension didn't appear it in Firefox, in a way that was obvious to me. Of course, not finding it may have been user error. It was assumed that there was some bug in the extension. Investigation was put to the side for the moment.

An icon did show up in Thunderbird, though.

The extension not giving evidence that it was installed, other than the Firefox add-on manger, and the icon in Thunderbird suggested something was wrong.

It was, I think, a natural conclusion that the Firefox extension was somehow affecting Thunderbird. The 'browser' in the option name and documentation was referring to web browsers.

After going through the obvious analysis and changes, and feeling that it represented something that shouldn't happen. I contacted the vendor. It didn't occur to me that it was related to an application feature. This post was put up because it felt to be a serious issue.

As it turns out, the option is not related to the extension, even though all evidence suggested a connection. It is a separate utility that attaches itself to what ever it thinks is a browser, and adds the icon to the menubar. Not just known browsers. There were imbedded links in the first emails that were opened. The icon disappeared if the email didn't have a link.

How it actually determines that something is a browser is unknown. i didn't try any other applications. The vendor suggested disabling the option. It was done and the icon went away. A utility that reports on startup changes flagged that a startup related to the product was removed.

It's not a Firefox or Thunderbird issue.

Thanks for your help.