I am running Safari (12.1.2) on an elderly version of Sierra (10.12.6) to maintain legacy software functionality. No more-recent version of Safari is available for the operating system installed.

Something is interfering with Safari. Here are some symptoms:

• Some site pages hang on login.

• A favourite Wordpress blog has occasional unpopulated picture frames and comment 'submit' buttons.

• In Stackexchange—I can login; but, I cannot comment, up or down vote. I can answer questions but the markdown hints are missing from the answer text box header. The answer submit button is ineffective. The site's top strip banner is crippled (clicking the hamburger takes me to StackOverflow not Meta).

SE support suggested using a different browser after they found no irregularities on the server side. It worked! Everything seemed normal on a different web client.

I am looking for a way (instructions, frankly) to fix Safari somehow without losing passwords, Top sites, bookmarks, etc. in the least destructive way possible

Edit: Cannot comment. How to create new user needed. Thanx.
2nd Edit: Done. Concur but cannot award answer : (