So, the background on this issue. I had two monitors. A 1080p Samsung monitor, and a 1440x900 Philips TV. On my old Radeon HD 5770, I connected the monitor via HDMI and the TV via VGA and set both to their native resolution and everything worked fine.
I switched to a 290x which no longer had any analog outputs, so now I have the monitor connected via HDMI and the TV connected via a DVI->HDMI converter. This only allows me to set the 1440x900 TV to one of four resolutions: 1920x1080, 1600x900, 1280x720 or 1024x768. So I set it to 1080p and let either my graphics card or the TV itself downscale it. I also turned off all scaling options in the windows control panel.
And this works, for everything except Firefox, which inexplicably doubles everything in size when the centre of the window is on the second monitor.
For illustration, here's a screenshot.
The left window is Firefox, on the primary monitor at a reasonable size. The rightmost window is Chrome, on the secondary monitor, also at a reasonable size. The middle window is Firefox, on the secondary monitor and doing some weird scaling of its own.
IS there any way to disable or fix this?