Firefox's peculiar handling of font choice preferences
On Twitter, I said:
I really don't understand how Firefox decides which particular 'Fonts for ...' preference controls the font sizes of any particular web page. They also seem to keep changing them.
If you look at Firefox's Preferences 'General' tab and scroll down, you'll discover a 'Fonts and Colors' preference which offers you the chance to set your default font and its size, and also has an 'Advanced...' option. As it happens, that default font is basically an illusion. If you go into the Advanced option, you'll discover that you can set fonts and font sizes for a whole range of, well, things (it's not quite languages). In particular you can set them for both 'Latin' and 'Other Writing Systems', and they can be different (and will be, if you only change one).
All of this matters because as far as I know there is no master preference that overrides everything. You cannot say 'in all things, I want my normal font to be X size and my monospace font to be Y size'. Instead you have to set this one by one in as many writing systems or languages as you think Firefox will ever decide web pages are in. If you miss one, your font sizes (and even font choices) can wind up weird and unsightly on some web pages.
On top of that, Firefox seems to sometimes change its view of what 'writing system' a particular web page is in even if nothing in particular changes in the web page itself. Wandering Thoughts has been serving pages with the exact same lack of language and writing system information for years, but I'm pretty sure that at some point some versions of Firefox flipped between using my 'Latin' font choices and my 'Other Writing Systems' ones (or, at the time, my non-choices for the latter; I hadn't set them). In addition, I don't think there's any easy way to see what font preference Firefox is using for any particular web page. You can see what character set encoding it thinks a page is in (and change it), but that's not the same thing as the 'writing system'.
(The writing system is also not the same thing as the language set in HTML attributes, but I suspect that Firefox generally derives the writing system from the language, if there is one.)
Unfortunately I suspect that none of this will ever really change. In the modern web, most websites explicitly set all of their font choices and font sizes, so these preferences are probably only ever used on a minority of websites, and the Firefox developers have higher priority work to do. Font preferences certainly wouldn't be the only bit of Firefox preferences and UI that are quietly being neglected that way.
(Why I care about this as much as I do is a discussion for another entry.)