User-Agent: Mozilla/5.0 (Windows; U; Windows NT 5.1; en-US; rv:1.9.0.6) Gecko/2009011913 Firefox/3.0.6 Build Identifier: Mozilla/5.0 (Windows; U; Windows NT 5.1; en-US; rv:1.9.1b3pre) Gecko/20090205 Shiretoko/3.1b3pre On everything except Windows, the layout.css.dpi pref will take -1 (default; logical or 96, whichever is higher), 0 (logical), or any positive integer. I don't see why this isn't possible on Windows. On a related note, would it be possible for Firefox to grab the DDC information itself rather than relying on Windows (which generally isn't set properly)? Reproducible: Always

Dupe of Bug 433664?

I don't think so, although this should probably either block or depend on it. I'll comment over there.

Any conclusion whether or not it is possible to support that preference in a meaningful way on Windows as well to either close or confirm this bug? There is also bug 537890 now on a redesign of the DPI system.

I tested 2.0.0.20, 3.0.18, 3.5.17 & 3.6.17 at 96, 120, 144 & 192 DPI on WinXP, both using those values directly for layout.css.dpi, as well as indirectly by setting it to 0, and in every case I got the results I expected loading http://fm.no-ip.com/Auth/dpi-screen-window.html. 4.0.1 works the same as the rest at 120, but fails shown by https://bugzilla.mozilla.org/attachment.cgi?id=530543. To me this means this bug was invalid when filed unless it broke somewhere between 2.0.0.20 and 3.0.6 and was fixed by 3.0.18.