My problem differs from #140540, because in all system fonts are quite good. I solved my problem. I just changed DPI in my Firefox and Thunderbird like this:
1. Оpen Edit/Settings/Settings/Advanced/Settings Editor in Thunderbird or write in address line about:config in Firefox.
2. Find layout.css.dpi
3. Set layout.css.dpi to 72
In my case for some strange reason there was -1.
PROBLEM SOLVED
My problem differs from #140540, because in all system fonts are quite good. I solved my problem. I just changed DPI in my Firefox and Thunderbird like this: Settings/ Advanced/ Settings Editor in Thunderbird or write in address line about:config in Firefox.
1. Оpen Edit/Settings/
2. Find layout.css.dpi
3. Set layout.css.dpi to 72
In my case for some strange reason there was -1.
PROBLEM SOLVED