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).
• Change the default value -1 to 1 to make it work like in previous Firefox versions (100%)

Start with a value of 1 and adjust this value with 0.1 steps or 0.05 for finer adjustments.

• Use values between 1.0 and about 0.5 to reduce elements in size (do NOT go all the way to 0.05!)
• Use values greater than 1.0 to magnify and make elements larger (percentage divided by DPI, % / DPI).
• http://kb.mozillazine.org/about:config

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

• Default FullZoom Level: https://addons.mozilla.org/firefox/addon/default-fullzoom-level/
• NoSquint: https://addons.mozilla.org/firefox/addon/nosquint/

Use this extension to adjust the font size for the user interface:

• Theme Font & Size Changer: https://addons.mozilla.org/firefox/addon/theme-font-size-changer/

I read this elsewhere, and tried it, but it is of no help to me.

The value it was at before I changed it was -1 for some reason, but anything under 0 had no effect. Anything above 0 made the entire browser ridiculously tiny.

try setting the preference to 1.0...

Again, it just goes too small. When I set it to anything between 0.0 and 1.0 it goes super small and just gets slightly bigger with each 0.1 added, but never reaches its original size.

It is best to adjust the layout.css.devPixelsPerPx pref to get the icon size in the user interface correct.

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

• Default FullZoom Level: https://addons.mozilla.org/firefox/addon/default-fullzoom-level/
• NoSquint: https://addons.mozilla.org/firefox/addon/nosquint/

Use this extension to adjust the font size for the user interface:

• Theme Font & Size Changer: https://addons.mozilla.org/firefox/addon/theme-font-size-changer/

• Big Buttons: https://addons.mozilla.org/firefox/addon/big-buttons/

I have since found out why. The new installation is actually a new version of Firefox, and this is a new 'feature' which it turns out quite a lot of people are complaining about. I'm going to wheather through it and just wait for Mozilla to issue a patch for it.

This is not a new feature.

The only change is that the value of the layout.css.devPixelsPerPx pref has been changed from 1 to -1 to make Firefox use the Windows DPI system settings. This pref has been around for quite a few years (2009).

Then why did this not happen on my previous install on the same machine?

Actually, found the setting needed on the forum where I saw everyone complaining about it.

The new '-1' is equivelant to a default of '1.25', setting the value to this fixed it all, and the dpi of websites, which any value between 0 and 1 did not change.