In general, if your software supports plugins, you can:

1. Have plugins with ultimate power (meaning that plugins can change everything, no encapsulation) 2. Stable API/ABI (meaning that plugins written for version 1 will work with version 2) 3. A refactorable code base.

You can pick only two out of three.

Traditionally, Firefox chose 1 and 2, and for stuck without multiprocess support for years (causing it to have worse performance and security), and now wants to move to 2 and 3.

Incidentally, I suspect that's _one_ of the reasons Linux doesn't have a stable Kernel ABI - they chose 1 and 3.