Extremely long timeouting (pending) XHRs when more than one FES UI Firestore open

### Environment info
**firebase-tools:** 9.1.0
**Platform:** MacOS 11.1
**Machine:** 3,3GHz Dual-Core Intel i7, 16GB, SSD
**Browser:** Google Chrome 87.0.4280.88 (Official Build) (x86_64)

### Test case
Not applicable

### Steps to reproduce
1. Run `firebase emulators:start --import=[DIR] --export-on-exit=[DIR]`
2. Open multiple tabs of Firestore in Firebase Emulator Suite UI
3. Call any firestore request. Read/write, frontend/backend
4. Observe pending and/or timeouting Firestore requests in Network tab

### Expected behavior
Non-pending/timeouting requests no matter the amount of tabs open.

### Actual behavior
Pending/timeouting requests