Firefox 41 has enabled the vertical writing support for East Asian scripts by default, adding the
writing-mode CSS property and various direction-independent
padding-related properties. Several existing properties have been unprefixed at the same time.
At the time of the Firefox 41 release, the serialized style rules including property names returned by
CSSRule.cssText, or iteration of the properties in a style rule, were the prefixed, legacy form, like
-moz-margin-start, rather than the unprefixed, canonical form, like
margin-inline-start. The Page Inspector’s CSS pane was also showing the prefixed properties even though unprefixed ones were already supported. This issue has been fixed with Firefox 42 by swapping the internal alias direction of the prefixed and unprefixed properties.
Those prefixed properties are deprecated and the support will be removed in the future.