Strange Mac add-on

I've attached a picture of a black box that pops up on a clients Macbook when he opens Firefox. Running 10.6.8. I have disabled all login items, I can find no extensions that would account for this.

This does not happen in Safari, I can't find any applications that look unusual. The box lists several items and grows longer, then after 15 seconds or so goes away. I cannot make it go away on my own.

I don't know what it is. Help?
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Bruce CorsonPresidentAsked:
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strungCommented:
Try following the instructions here to remove the AddThis plug-in from Firefox:

http://support.addthis.com/customer/portal/articles/381217-add-ons-and-toolbars#.Us3dK3n000o

Go to the Tools menu of your Firefox browser, select Uninstall... from the AddThis sub-menu.

Or, you can also uninstall the AddThis plug-in by selecting Add-ons from the Firefox Tools menu. Then, select Extensions and click the uninstall button under AddThis. Be sure to restart your browser for the change to take effect.

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Bruce CorsonPresidentAuthor Commented:
Thank you, I'll try that. I won't have access to the client's computer for a couple days, so I'm going to leave this open. STRUNG, are you sure this "uninstall" applies to Mac versions of Firefox as well?
strungCommented:
Good point. I suspect it is probably the same. I can't check the first option (of going to the Tools>AddThis menu since I don't have AddThis installed, but I suspect that that should be the same.

Looking at my Mac copy of Firefox, it looks like the second option should be much the same as described.  The tools menu has an "Add-ons Manager" which allows you to delete or disable or remove extensions and disable plug-ins.
strungCommented:
Also, in Firefox you can go to:

Help>Troubleshooting Information>Application Basics>about:plugins

and you will be given a list of installed plug ins and their location in Finder. You can use that information to delete the plug in manually.
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