No, you do not need a subscription to ACI Scholarly Blog Index in order to install and use the ACI Blog Index for Search Engines browser extension. However, the extension may link to articles in ACI that require further access to view the full text.
An affordable individual subscription will provide full access to articles in the ACI database. See current pricing to create an account then check your email to set up a password. Search activity in the browser extension is anonymized and is not associated with your ACI account even when you are logged in to ACI.
Access provided by your institution
Many colleges, universities, and libraries provide access to ACI on campus. In most cases this is directed toward specific on-campus computers by IP address or to any email addresses @youruniversity.edu. Please contact your librarian for information about accessing ACI.