Bring back compatibility with XPI add-ons

[endolith]

I'm still running Thunderbird 52.9.1, because I depend on add-ons like Junquilla and AttachmentExtractor, which can't exist in modern versions of Thunderbird. 

I still run Waterfox Classic, because I need add-ons like DownThemAll and Mozilla Archive Format, which can't exist in modern versions of Firefox.

These aren't great solutions, but they're better than the current versions of each product.  I wish I didn't have to choose, and the modern versions of software supported powerful third-party customization like the old ones.

The golden era of Mozilla was when the products were minimal, and depended on third-party add-ons to provide all extra functionality.  That's what made Mozilla products popular, and this strategy required less development effort by Mozilla.  The switch to Chrome extensions was the worst development in Mozilla history, making the products much less useful, and I've been waiting for years for you to bring back customization of the browser/mail client.

[Jon]

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