Do you have the same issue under a different user? Have you tried downloading a separate copy of Firefox to a folder and run it to see if you have the same issue. https://www.mozilla.org/en-US/firefox/all/#product-desktop-release

You can also run another version to verify. I currently have 3 versions plus the OS installed version. http://ftp.mozilla.org/pub/firefox/releases/

It would help if you provided a link to the article you are referring to. That way we know what steps you have already tried. Is the slowness on any site?

A fresh copy gives the same problem. Examples: If I type the quick brown fox jumps over the lazy dog in the search feield, I finish typing when the field displays "jump". It takes 3-5 more seconds to finish. In 87 it was instantaneous. When typing my 12 characters password, the screen displays the first character by the time I've finished. Before: instantaneous. Same if I use backspace to delete characters. It lags about 4 seconds. Loading pages is also generally slow. For any page. The articles that do not help are here: https://support.mozilla.org/en-US/products/firefox/fix-problems/slowness-or-hanging Nothing of this happened in 87. I can happily wait until 89, but it is annoying that (a) I cannot automatically keep my setup (b) mozilla keeps urging me to update

How about a different user on the same system?

It's only me in the system, who has had successful company of firefox during 87 releases. I guess the problem lies in firefox, not in the user. It behaves as if it is working against the disk rather than against RAM.

Firefox 89 is as slow as 88. I tried: fresh start, new user, no add-ons, refresh firefox, all without result. Running firefox 87 on Linux 64 Debian 10, it responds instantaneously. With 88 or 89, it gets slow. There is a many-seconds delay between typing and getting response on screen.

What can I do?