Firefox seems to have some threshold to leap to a larger font or a different font rendering scheme when adjusting the (Font) Scaling Factor in Ubuntu Tweaks. Nothing happens in Firefox windows until the Scaling Factor reaches 1.25 (125%) and then it jumps to render larger fonts everywhere. (next jump at 1.4) See the screenshots for the difference. What is this threshold? What is the actual parameter Firefox reacts to? Can I control it manually within Firefox? This question has a rather convoluted background: I have replaced my 2560x1600 monitor with the 3840x1600 monitor and suddenly all Firefox windows render super ugly. It's not that they just look smaller due to to the smaller PPI but rather their font shape causes my eyes to tear. Previously the sensation was very soft. I do not know what that is but it seems it's not just the size. Could it be Firefox is using some information from the monitor that brings it to a sub-optimal font rendering/hinting/antialiasing behavior? Increasing (Font) Scaling Factor to 1.25 via "Ubuntu (Gnome) Tweaks" app makes the fonts look acceptable again. Ubuntu Scaling Factor obviously also changes the size of other windows and their fonts. So I would rather keep the Scaling at 100% and just make sure the fonts are rendered in Firefox as before with the old monitor. I.e. I would like to remove this quality regression. The fonts can be controlled via Firefox properties but as expected this either increases all font sizes or doesn't work for proprietary fonts supplied by web pages (min font size). There should be a better solution than tweaking each font individually. The fact that the fonts are suddenly rendered acceptably again after the Scaling Factor reaches 1.25 makes me think that there may be some threshold in Firefox that makes it use different rendering scheme, mechanism or such. After 1.25 nothing happens until Scaling Factor reaches 1.4, then the font increases again. So can I just make Firefox render fonts as when scaling is at 1.25 but with the actual scaling factor at 1.0? Apart from the overall size/resolution the monitors (old Samsung 305T vs. new Dell u3821dw) have different pixel pitch 0.2505 mm, PPI 101 (old) vs. 0.22908 and PPI 110,87 (new). Can this this be the root cause of the ugly fonts? So is there maybe a different parameter or setting to tell Firefox to render fonts regardless the higher PPI or lower pixel pitch? I would expect everything to look just a bit smaller due to a smaller pixel pitch, but instead the fonts are rendered completely different and I can't find the setting that would fix that. Just FYI I have been playing with these settings: Firefox: - Default Zoom 120% - Check Zoom Text Only - Minimal font size - layout.css.devPixelsPerPx 1 - gfx.font_rendering.cleartype_params.force_gdi_classic_max_size 100 Ubuntu: - Scaling Factor - System Fonts I'm on Ubuntu 18.04 using Firefox 89.0 Ubuntu canonical-1.0 .