I did a refresh of Firefox and the default now is 16px as it should be. Have no idea where 12px came from. Also selected default font was unknown to me. Now input fields are still a little too large compared to other browser on Ubuntu and Firefox Windows, but the problem with malfunctioning website is disappeared with current default. So question can be closed.

Hi GertKraats, you can mark your reply as the Solution.

For future reference, you might be able to spot a font discrepancy as follows:

(1) In a new tab, type or paste about:config in the address bar and press Enter/Return. Click the button promising to be careful.

(2) In the search box above the list, type or paste font*size and pause while the list is filtered (for example, to show font.size.variable.x-western).

If any are customized to a smaller size than expected -- these usually appear bolded and have a status of "user set" -- you can right-click > Reset those values.

(3) In the search box above the list, type or paste font.name and pause while the list is filtered (for example, to show font.name.serif.x-western).

If any are customized to an unknown or unwanted font family -- these usually appear bolded and have a status of "user set" -- you can right-click > Reset those values.

If the problem returns on its own, an extension may be modifying the values. Some Linux distributions bundle in theming extensions. You can try disabling those on the Add-ons page. Either:

• Ctrl+Shift+a
• "3-bar" menu button (or Tools menu) > Add-ons

In the left column, click Extensions. Then cast a critical eye over the list on the right for anything nonessential or unrecognized. I suggest disabling everything you don't recognize and/or do not plan to use for the next 48 hours.

Often a link will appear above at least one disabled extension to restart Firefox. You can complete your work on the tab and click one of the links as the last step.

Thanks a lot. Will investigate Firefox at Windows and adjust if necessary