OK, it seems that some sites (a few google analytics properties?) are acting as if they only support the new key exchange---they are sending hello retry requests to force its use. I guess those connections were just failing previously. I'm going to mark this as a duplicate of Bug 1874962, since the patch there will fix the assertion failure. It's queued for landing now.
Bug 1875098 Comment 3 Edit History
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OK, it seems that some sites (a few google analytics properties?) are acting as if they only support the new key exchange---they are sending hello retry requests to force its use. I'm going to mark this as a duplicate of Bug 1874962, since the patch there will fix the assertion failure. It's queued for landing now.