Browsers non accessing http websites

Hi, all at once my two browsers (Safari and Firefox) cannot access anymore several http websites that I usually access, for instance
Yet some other http sites get loaded correctly (showing “Not Secure” in the bar).

The only unusual things I did a couple of days ago were to install app (by now removed) and reset my Mac with PRAM because the internal microphone would not show in System Preferences > Sound Input. But even after that I could access http websites. Now Safari & Firefox, after a certain time, say that such sites do not respond.

I restarted my MBP several times and run DiskDoctor too, but nothing happened.
By the way: running the last Catalina (updated months ago).

Is it safe to copy/paste my previous user library from my backup external disk?
Any suggestion most welcome. Thank you.

Try clearing your DNS cache.

From a terminal session type
sudo dscacheutil -flushcache; sudo killall -HUP mDNSResponder

You will be asked to provide your password.

Well well well. Browsers accessing http correctly, and it seems more quickly then before.
Thank you so much.