A small History of browser support for CSS variables

Firefox support for CSS variables began with version 29 (with the var- prefix, released 29.04.2014) and as an option to be turned on in about:config for this to work.

Support without the custom prefix exists since version 31 (22.07.2014) and with the about:config option turned on by default.

The about:config preference to toggle CSS variables was finally removed in Gecko 55 (08.08.2017), which is the current version; it includes Firefox 55 and other browsers that use the current Gecko rendering engine.

CSS variables have been supported in Google Chrome from the outset with the -webkit-var- prefix and when 'Experimental Web Features' were turned on in chrome://flags.

Prefixed variables functionality existed in Chrome until 33.0.1750 (20.02.2014, Blink 537.36, V8 v3.23.17), and was removed in Chrome 34.0.1847 (08.04.2014, Blink 537.36, V8 v3.24.35) because of performance issues. V8, or Chrome V8, is the JavaScript engine used in Chrome.

Un-prefixed variables were implemented in Chrome 49.0.2623 (02.03.2016, Blink 537.36, V8 v4.9.385). The current Chrome version is 61.0.3163 (05.09.2017, Blink 537.36, V8 v6.1.534).