Attachment didn't fly first time

Thank you for the screen shot. It mentions a script pulled from the icloud.com website. But this happens on lots of other websites with no relation to Apple?

Could you check on whether you have any iCloud related extensions that could be embedding that script? Open the Add-ons page using either:

• Cmd+Shift+a
• "3-bar" menu button (or Tools menu) > Add-ons

In the left column, click Extensions. Then check on the right side. Bear in mind that all extensions are optional; none come with Firefox. If in doubt, disable.

Often a link will appear above at least one disabled extension to restart Firefox. You can complete your work on the tab and click one of the links as the last step.

Does that help?

There are only two extension listed and NEITHER are enabled

Is there any pattern to websites which generate that dialog? Or perhaps iCloud was just on that site, and you get other slow scripts on other sites.

Here is our general support article on unresponsive script errors if you haven't seen it yet: Warning Unresponsive script - What it means and how to fix it.

Script Error only seems to happen when using Mail in iCloud. Other browser apps do run much slower than "normal"