I'm sure the support crew would like to have a more complete description of what occurs when you attempt to attach a file -- e.g., does it cause FF to seize, or simply not respond to whatever command or click-action used in the attempt?

I've occasionally had a similar problem associated with this e-mail attachment function or the action to select a new file-download location, but the e-mail attachment issue does not arise with the current releases -- all reflected in this bugzilla report:

https://bugzilla.mozilla.org/show_bug.cgi?id=1046958

When I try to send the email and it doesn't go through I get the error message 500 internal server error and it does not save as draft or anything.

I know you said the problem didn't occur by using IE, but do you have the option of using a different web-based e-mail provider to ascertain whether that same behavior occurs and confirm or exclude FF as the issue source in conjunction with that mail provider's interface?

Have you made any contacts with the mail provider about this issue? If your provider's interface has recently incorporated ActiveX controls which FF can't/won't process, IE will work just fine because it can process those background scripting procedures.

yes, the problem also happens using AOL with Mozilla.