obviously it's too late, but it's a good practice to keep the 3rd party dependencies mirrored in your own infrastructure :) There is NO GUARANTEE that even a huge site (like launchpad for downloading DEBs) won't go down over a period of time.

But it has as an external service several disadvantages which opposed the philosophy of static websites diametrically.

Add-ons must be self-contained and not load remote code for execution

Unfortunately, loading their code from their CDN is the only way those services permit their use.

Mozilla does not permit extensions distributed through https://addons.mozilla.org/ to load external scripts. Mozilla does allow extensions to be externally distributed, but https://addons.mozilla.org/ is how most people discover extensions. The are still concerns: Google and Microsoft do not grant permission for others to distribute their "widget" scripts. Google's and Microsoft's "widget" scripts are minified. This prevents Mozilla's reviewers from being able to easily evaluate the code that is being distributed. Mozilla can reject an extension for this. Even if an extension author self-distributes, Mozilla can request the source code for the extension and halt its distribution for the same reason.