Today, while I was in the middle of using my normal browser instance, it decided to switch from DejaVu Sans to Noto Sans as my default font :

Dear Firefox: why are you using Noto Sans all of a sudden? I have you set to DejaVu Sans (and DejaVu everything), and fc-match 'sans' and fc-match serif both say they're DejaVu (and give the DejaVu TTF files). This is my angry face.

This is a quite noticeable change for me because it changes the font I see on Wandering Thoughts , my start page , and other things that don't set any sort of explicit font. I don't like how Noto Sans looks and I want DejaVu Sans.

(I found out that it was specifically Noto Sans that Firefox was using all of a sudden through the Web Developer tools 'Font' information , and confirmed that Firefox should still be using DejaVu through the way to see this in Settings .)

After some flailing around , it appears that what I needed to do to fix this was explicitly set about:config's font.name.serif.x-western, font.name.sans-serif.x-western, and font.name.monospace.x-western to specific values instead of leaving them set to nothing, which seems to have caused Firefox to arrive on Noto Sans through some mysterious process (since the generic system font name 'sans' was still mapping to DejaVu Sans). I don't know if these are exposed through the Fonts advanced options in Settings → General, which are (still) confusing in general . It's possible that these are what are used for 'Latin'.

(I used to be using the default 'sans', 'serif', and 'monospace' font names that cascaded through to the DejaVu family. Now I've specifically set everything to the DejaVu set, because if something in Fedora or Firefox decides that the default mapping should be different, I don't want Firefox to follow it, I want it to stay with DejaVu.)

I don't know why Firefox would suddenly decide these pages are 'western' instead of 'unicode'; all of them are served as or labeled as UTF-8, and nothing about that has changed recently. Unfortunately, as far as I know there's no way to get Firefox to tell you what font.name preference name it used to pick (default) fonts for a HTML document. When it sends HTTP 304 Not Modified responses, Wandering Thoughts doesn't include a Content-Type header (with the UTF-8 character set), but as far as I know that's a standard behavior and browsers presumably cope with it.

(Firefox does see 'Noto Sans' as a system UI font, which it uses on things like HTML form buttons, so it didn't come from nowhere.)

It makes me sad that Firefox continues to have no global default font choice. You can set 'Unicode' but as I've just seen, this doesn't make what you set there the default for unset font preferences, and the only way to find out what unset font preferences you have is to inspect about:config.

PS: For people who aren't aware of this, it's possible for Firefox to forget some of your about:config preferences . Working around this probably requires using Firefox policies ( via ), which can force-set arbitrary about:config preferences (among other things).