It would be cool to make the default HTML font size be dependent on the default UI font size, since people who set their UI font to tiny probably want tiny fonts on the web, and people who enlarge their UI font probably do so because they need bigger fonts, and they'll need bigger fonts on the web too. I see several possibilities: a) Make font.size.*.* be a percent ratio wrt the default UI font size (or where applicable on the corresponding UI font size; on gtk I think all serif, sans-serif, etc, can be queried). The problem is that it suppresses the possibility to specify an absolute size in pixels. b) Make font-size.*.* accept a special value "0" which would mean calculate from ui font. The pb is that the ratio is then hardcoded (probably not to 100%, since on IE the default font is bigger than the ui font and most websites specify fonts accordingly) c) Make font-size.*.* treat negative values as the opposite of the ratio, or create another pref. This bug is filed in Firefox, since if the feature is not wanted by UI people it doesn't make much sense to implement it in Gecko.
Bug #414427 is somewhat similar but more specific Absolute pixel size is a dead end IMHO and if this is needed it should be done at the desktop level and all the firefox-level font prefs overrides need to die die die.
(In reply to comment #1) (ie use desktop font sizes exposed by XSettings, do not try to invent something else that will be inconsistent with the rest of the system and just confuse and annoy users)