Why does firefox (not esr, from flatpak) on Debian 10 KDE have poor font rendering compared to other browsers?

Question

I switched to Debian from Kubuntu. I fixed the fonts in the system, but there are bad ones in the browser.

I have a live usb image of fedora. I started with it and watched the work of firefox. The fonts render fine, but the font settings in the browser are identical.

How to fix it?
Thank you.

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Update:
Could it have something to do with proprietary software that is not used in debian by default?

question from:https://stackoverflow.com/questions/65893444/why-does-firefox-not-esr-from-flatpak-on-debian-10-kde-have-poor-font-renderi

Answer 1

Waitting for answers