My Vista x86 has the August WS2008 IE9 patch KB4034733 installed. In the IE9 brower, the dialogue box for Tools | Internet Options | Advanced > Security also has only 'Use TLS 1.0' listed and the box is checked (i.e., no listing for TLS 1.1 or TLS 1.2). (It also has the box for 'Use SSL 3.0' checked. I am thinking that I should uncheck this box, because otherwise, TSL 1.0 can downgrade to SSL 3.0 and thus, weaken security, were I to use the IE9 browser. Any ideas about this?
Yes, I would recommend disabling SSL 3.0 support in your IE9 browser. I did this back in 2014 when the POODLE exploit was first reported - see Martin Brinkmann's October 2014 ghacks.net article SSL 3.0 Vulnerability Discovered. Find Out How to Protect Yourself. Both Mozilla and Google automatically disabled SSL 3.0 support in their browsers in January 2015 (FF v35, Chrome v40), and when it was later discovered that TLS 1.0 was also vulnerable to the POODLE exploit I went ahead and turned off TLS 1.0 in my Firefox settings as instructed in the SSL.com article Turn Off SSL 3.0 and TLS 1.0 in Your Browser. Unfortunately, you can't disable TLS 1.0 in IE9 since you need at least one supported protocol to make a secure connection.
I switched from IE9 to Firefox back in 2011 because of this lack of TLS 1.1 and 1.2 support in IE9. Firefox ESR is now the only secure browser for use on Vista SP2 computers since Google stopped supporting the Win XP and Vista platforms in April 2016 per their announcement <here>.
BTW, I read on Wikipedia (about Transport Layer Security) that Firefox enabled TLS 1.3 by default in Feb, 2017. But, given your brower test, this must not include Firefox ESR??
According to Martin Brinkmann's June 2017 ghacks.net article How to Enable TLS 1.3 Support in Firefox and Chrome "Both Firefox and Chrome support TLS 1.3, but the version of Transport Layer Security is not enabled by default. The main reason for that, likely, is that it is still only available as a draft." TLS 1.2 is still the recommended protocol until TLS 1.3 comes out of beta. I posted further information about IE9 and TLS 1.3 in the Norton Tech Outpost board <here>.
32-bit Vista Home Premium SP2 * Firefox ESR v52.3.0 * NS Premium v126.96.36.199 * MB Premium v3.1.2
A super thanks for all of your tips here and that ssllabs.com url for testing browsers in a previous post.