Minimum permissions required to run sp_Blitz
Yes, it requires sa permissions. But since it's a stored procedure, you can get permissions via a certificate

See the section How to Grant Permissions to Non-DBAs

USE master;
GO
CREATE CERTIFICATE sp_BlitzFirst_cert
ENCRYPTION BY PASSWORD = '5OClockSomewhere'
WITH SUBJECT = 'Certificate for sp_BlitzFirst',
START_DATE = '20130711', EXPIRY_DATE = '21000101';
GO
CREATE LOGIN sp_BlitzFirst_login FROM CERTIFICATE sp_BlitzFirst_cert;
GO
CREATE USER sp_BlitzFirst_login FROM CERTIFICATE sp_BlitzFirst_cert;
GO
GRANT EXECUTE ON dbo.sp_BlitzFirst TO sp_BlitzFirst_login;
GO
GRANT CONTROL SERVER TO sp_BlitzFirst_login;
GO
ADD SIGNATURE TO sp_BlitzFirst BY CERTIFICATE sp_BlitzFirst_cert
WITH PASSWORD = '5OClockSomewhere';
GO
GRANT EXECUTE ON dbo.sp_BlitzFirst TO [public];
GO


Then whenever you update the proc, you’ll need to reassign the permissions by adding the signature again:

ADD SIGNATURE TO sp_BlitzFirst BY CERTIFICATE sp_BlitzFirst_cert
WITH PASSWORD = 'Get lucky';
GO
GRANT EXECUTE ON dbo.sp_BlitzFirst TO [public];
GO


Hope this helps!