Why Drop IE Support?
For web libraries and apps, maintaining Internet Explorer support in addition to support for modern browsers has long required a lot of additional code or code compromises that often lead to larger bundle sizes or prevent using new useful language features.
Microsoft itself ended support for IE11 in its Microsoft 365 apps and services as of August 2021 and is dropping support for the Internet Explorer desktop app itself on June 15, 2022, encouraging users to upgrade to Edge.
By no longer having to support IE, the Firebase JS SDK will be able to take advantage of the latest features from various libraries, including ESM exports, which allow us to make Firebase JS SDK more compatible with ESM build tools.
Additionally, there may be bundle size savings here and there from eliminating code branches that only exist to deal with IE special cases.
For Users Who Still Need IE Support
If you still need to use Firebase with IE support, you can continue to use any version less than 9.8.0. Unfortunately, we cannot continue to update these earlier versions with bug fixes.
Installing your own polyfills can cover some functionality differences, but we will not maintain an ongoing list of polyfills required to support IE.
Feel free to contact us with any issues or feedback by filing an issue on the Firebase JS SDK Github repo.