Type about:config into the Firefox address bar and tap on the Enter key.
If the warning This might void your warranty appears, click I'll be careful, I promise.
Search for layout.css.devPixelsPerPx
Double-click on layout.css.devPixelsPerPx to modify its value. The default is -1.0 in Firefox 22 and above. Change it to 1.0 to make it work like in previous Firefox versions.
If necessary, further adjust the value in 0.1 or 0.05 steps. Use values between 1.0 and about 0.5 to reduce elements in size. Use values greater than 1.0 to increase size. For example, a value of 1.25 will increase font size to 125% to accommodate the default DPI setting in Windows 8. Double check the value that you enter. Setting a too small value will make everything disappear and a too large value will blow things up.