Well, as the article said, there is an user-selectable option, so the answer to "Why not allow users to decide themselves?" is "they did"
As for default, that's a different question unrelated to user freedom. Unfortunately in Windows, DLL injection is very often abused for no good reason. Imagine a simple, non-technical user getting asked about permitting mouse driver DLL.. what would user think? With only information being DLL name/description, they might think that mouse will not work without it and permit it (see https://news.ycombinator.com/item?id=36681873 for real reason). And once firefox starts crashing, will they know how to disable it?
No, it is much safer to disable it all by default. And if a powrer user needs a specific DLL, they can enable it. As a bonus, they will learn where the dialog is, so they can disable it back once the crashes start.
As for default, that's a different question unrelated to user freedom. Unfortunately in Windows, DLL injection is very often abused for no good reason. Imagine a simple, non-technical user getting asked about permitting mouse driver DLL.. what would user think? With only information being DLL name/description, they might think that mouse will not work without it and permit it (see https://news.ycombinator.com/item?id=36681873 for real reason). And once firefox starts crashing, will they know how to disable it?
No, it is much safer to disable it all by default. And if a powrer user needs a specific DLL, they can enable it. As a bonus, they will learn where the dialog is, so they can disable it back once the crashes start.