Sounds like paranoia to me. And you are planning to do serious stuff on such ancient machine?
That post was from a little while ago and now is no longer relevant, since Firefox Legacy was updated this week to 68 ESR, which is up to date as of August 25, 2020.
I'm only moderately concerned about security on these old machines. I'm mostly concerned about what the banks think about the security of these browsers, and AFAIK typically they will support browser versions at least 2-3 years old. That means my 2008 MacBook4,1 running 10.7 Lion is probably good until 2022 or so. That is the extent of the "serious stuff" I'm personally interested in on these old machines.
Meanwhile, I'm typing this post on a 2007 MacPro2,1 which is officially supported on 10.7 Lion, and unofficially supported on 10.11 El Capitan. On this machine, I have full Chrome support, and full support for Firefox 78 ESR which is the latest ESR version. I suspect Firefox 78 ESR will continue to be supported until mid to late 2021, which means this machine is good for bank access until at least 2023 or so, and probably much longer if ParrotGeek brings the future 88 ESR version to El Capitan. I suspect 88 ESR (or whatever the number will be) will be available in mid 2021, and will be supported until mid to late 2022, meaning this machine could still be viable with the banks until 2024-2025
if ParrotGeek can continue its excellent work with these browser backports. 👍
OTOH, both my 2008 MacBook5,1 and my 2009 MacBookPro5,5 are running Catalina.
Too bad Apple didn't support these Mac Pros longer, because this Mac Pro is literally 4X as fast as those laptops, and I've got a glorious 30" Cinema Display hooked up to it too.