That is caused by code for HiDPI that landed.

You can modify the layout.css.devPixelsPerPx and increase or decrease the value in 0.1 or 0.05 steps to adjust the size of fonts and other elements in Firefox.

• The layout.css.devPixelsPerPx pref is a String value parsed to a float and allows to fine tune the dimensions of all elements (user interface and web pages) more precisely (resolution 0.1 or 0.05).
• You can use values between 0.05 and 1.0 to reduce elements in size or use values greater than 1.0 to magnify and make elements larger (percentage divided by DPI, % / DPI).

If the web pages needs to be adjusted after changing this pref then you can look at Default FullZoom Level or NoSquint.

• Default FullZoom Level - https://addons.mozilla.org/firefox/addon/6965
• NoSquint - https://addons.mozilla.org/firefox/addon/2592

See also this MozillaZine forum thread.

Does Fx 22 nightly look too large?:

• http://forums.mozillazine.org/viewtopic.php?f=23&t=2678897

Thank you. Does it help with the zoomed in UI too?Also i used to download my wallpapers from wallpaperswide. It has stopped detecting my resolution. It doesn't highlight it anymore.Similarly for some other websites they don't scale to fit the page anymore.In many pages the text is out of proportion. Also is there any upcoming solution from Mozilla team or is it here to stay?

Thank you. Does it help with the zommed in UI too?Also i used to download my wallpapers from wallpaperswide.It has stopped detecting my resolution.it doesnt highlight it anymore.Similarly for some other websites they dont scale to fit the page anymore.In many pages the text is out of proportion. Also is there any upcoming solution from mozilla team or is it here to stay?

The pref layout.css.devPixelsPerPx has effect for everything in Firefox (web page area and user interface).
So if you use this pref then you may have to (or want to) correct the zoom for the browsing area that displays internet content with one of the mentioned extension (Default FullZoom Level or NoSquint).