There's no way to fix issues that run that deep without breaking backwards compatibility with all the existing code written for those platforms, which would negate the entire point of the exercise.
No, it was in the 2000's and 2010's. And even more than 10 years ago, people were aware of how Internet can be dangerous, as shown by Internet Explorer.
Moreover, Firefox had sandboxing before Chrom* and Electrolysis:
1. https://developer.mozilla.org/en-US/docs/Archive/Add-ons/Sec...
2. https://developer.mozilla.org/en-US/docs/Archive/Add-ons/Dis...
3. https://developer.mozilla.org/en-US/docs/Archive/Add-ons/Int...