I haven't seen that one. I assume you're using the standard UI, not the "light" UI?

When I click the icon, I get a dialog pointed to my Documents folder. I hardly have any files in there, so if the quantity of files listed in the dialog is the issue, I can't replicate it. Do you have 500+ documents in the top level of that folder (or whatever folder the paper clip defaults to)? If so, can you work around the issue by moving some of them into a subdirectory?

Another thought: when you have a problem with one particular site, a good "first thing to try" is clearing your Firefox cache and deleting your saved cookies for the site.

1. Clear Firefox's Cache

orange Firefox button or Tools menu > Options > Advanced

On the Network mini-tab > Offline Storage : "Clear Now"

2. If needed, delete the site's cookies here

While viewing a page on the site, right-click and choose View Page Info > Security > "View Cookies"

This is likely to log you out. After logging back in, does the attachment feature behave correctly?

Yes, I am using the standard UI. I tried your idea of moving the files in the My Documents folder into a subdirectory but the problem persists. It happens as soon as I click on the paperclip icon. This new "feature" began after Windows 7 ( and all my other software) was reinstalled on my laptop after a hard drive failure, and if I use IE the problem doesn't occur - though if I google the error message, there is evidence that people have had the same issue with IE at times

Thanks for your second suggestion. I cleared the cache and cookies, but the problem continued...until I closed and restarted Firefox. After I did that the problem was gone. So, thanks very much for your help!