I ran into this and was practically beating my head against my desk in frustration until I realized: I didn't configure the dpi for my monitor. Odd thing was, which let me to not think of that: Konqueror was displaying pages just fine, with the proper proportions (!!) and Gecko-based browsers (Mozilla, Firefox) were not!
Measure your monitor in inches and divide your resolution (x and y) by the dimensions - then run
Look for a line which resembles the following:
dimensions: 3840x1440 pixels (1084x406 millimeters)
resolution: 90x90 dots per inch
If the DPI does not come within a few dpi of what you calculated, you can either edit the xf86config file manually, or run XF86Config or SaX/SaX2 (depending on distribution) to set the size of the monitor. Most of the scripts will ask for the size of the monitor(s) in mm, so be sure to measure the visible width and height of each monitor.
If configuring the font sizes in Firefox didn't help, this probably will.