Firefox Quantum and Web Extensions
With the introduction of Firefox Quantum, Mozilla removed support for legacy XUL extensions in favor of Web Extensions. This naturally made a lot of people angry. XUL extensions had full control over the browser and could change everything from how downloads where handled to the user interface. This powerful framework gave Firefox what many seen as a competitive edge. Extensions this powerful came with some shortcomings, however, and this lead to the adoption of Web Extensions in Firefox.
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