fix: browser detection #12435

Job	Run time
Build Firestore	10m 20s
Build Firestore	10m 20s
Test Firestore (test:browser)	7m 59s
Test Firestore (test:browser)	7m 0s
Test Firestore (test:travis)	3m 58s
Test Firestore (test:travis)	3m 58s
Test Firestore Compatible	4m 37s
Test Firestore Compatible	4m 37s
Test Firestore on Firefox (test:browser)	8m 17s
Test Firestore on Firefox (test:browser)	8m 17s
Test Firestore (test:lite:browser)	1m 30s
Test Firestore (test:lite:browser)	1m 30s
Test Firestore Compatible on Firefox	5m 35s
Test Firestore Compatible on Firefox	5m 35s
Test Firestore on Firefox (test:travis)	3m 37s
Test Firestore on Firefox (test:travis)	3m 37s
Test Firestore (test:browser:prod:nameddb)	7m 44s
Test Firestore (test:browser:prod:nameddb)	7m 44s
Test Firestore on Firefox (test:lite:browser)	1m 33s
Test Firestore on Firefox (test:lite:browser)	1m 33s
Test Firestore (test:lite:browser:nameddb)	1m 20s
Test Firestore (test:lite:browser:nameddb)	1m 20s
Test Nightly Firestore with Chrome	0s
Test Nightly Firestore with Chrome	0s
Test Firestore on Firefox (test:browser:prod:nameddb)	8m 9s
Test Firestore on Firefox (test:browser:prod:nameddb)	8m 9s
Test Firestore on Firefox (test:lite:browser:nameddb)	1m 27s
Test Firestore on Firefox (test:lite:browser:nameddb)	1m 27s
Check all required tests results	1s
Check all required tests results	1s
	2h 11m 15s

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