2019 MacBook Pro user, they made me download and run chrome to take the exam, last time (July-flex for me) they let me use Firefox. I am not sure if that's even a choice now, so have Chrome as an option just in case!
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"Starting with the September 2019 test administration, test takers will ... decision that the May, June, July, and August 2020 LSAT-Flex ... . Tests taken prior to September 2019 will not count against these ...