what is the best browser to replace safari browser 15.6.1 on Catalina

For older Mac struggling to make secure connections with an older Browser, FireFox has their "extended support edition". It is intended to last longer despite the built-in Browser getting too old.

https://support.mozilla.org/en-US/kb/switch-to-firefox-extended-support-release-esr

SquarePin wrote:

It is an old machine. But I still need to use it. What browser will work on the 15.6.1 Catalina

I would consider always having more than one browsers...there are many

personally FireFox has been my main working browser:

https://support.mozilla.org/en-US/questions/1467773

iCab, Firefox, or Waterfox. If it doesn't render, then the website probably isn't secure anyway.