Test

Add @types/firefox-webext-browser #2001

Job	Run time
tslint	40s
test (ubuntu, firefox, stable)	2m 30s
test (ubuntu, firefox, nightly)	2m 22s
test (ubuntu, chrome, stable)	1m 57s
test (ubuntu, chrome, nightly)	1m 55s
test (macos, firefox, stable)	5m 51s
test (macos, firefox, nightly)	1m 32s
test (macos, chrome, stable)	3m 47s
test (macos, chrome, nightly)	31s
test (windows, firefox, stable)	7m 36s
test (windows, firefox, nightly)	6m 54s
test (windows, chrome, stable)	6m 25s
test (windows, chrome, nightly)	6m 13s
	48m 13s

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