The Developer Tools use at least 3 different monospace font families for code/monospace text. See the attached screenshot for an example in Firefox 24 on OSX, with a funky-looking user-defined font for web page text (that's font number 2 in the screenshot). I’d argue that using a single font family makes more design sense. Also, users might be puzzled if their pref for monospace fonts (from “Preferences > Content > Fonts & Colors > Monospace”) that usually apply to Web content also alter the Devtools UI. (Although the source view in FF has always done that.)

After updating to FF 25.0a2, I’m only seeing 2 fonts instead of 3. - Console: seems to be using the system default (looks like Menlo in my case). - Inspector, Debugger, Style Editor: seem to be using the user pref (with a fixed size in Inspector, and the user-defined size in Debugger and Style Editor). It might still be a good thing to use the same font (from the same source) in Console and the other tools.

This patch makes devtools use only one monospace font. Any element that had a monospace font now gets the "devtools-monospace" class. I didn't touch the source editor, the two fonts happen to be in sync right now though. In addition, the CSS autocomplete popup now uses a monospace font. The monospace font size was increased a bit on OS X, after many (a ton) of complaints about it being too small. It was decreased a bit on Linux. It looks on par with the source editor now. Try builds: http://ftp.mozilla.org/pub/mozilla.org/firefox/try-builds/fayearthur@gmail.com-77304d6970af

Try tests running: https://tbpl.mozilla.org/?tree=Try&rev=e4ac1844ed66