Having Firefox 81.0.2 under Kubuntu 18 I would like to set bigger default font size for
it and I found fonts options : https://prnt.sc/vjjotn
but setting big value as default size, but it does not applied.
On opening firefox has small fonts.
How it can be fixed?
Your preferences have been applied but the web pages are setting their own font size which is over-riding your font size setting in the browser. If you want your settings to over-ride page settings, click
Advanced..., you will see a dialog box similar to the one in the picture below and then uncheck
Allow pages to choose their own fonts, instead of your selections above checkbox. Then save your settings.