diff --git a/includes/sanitizers/class-amp-style-sanitizer.php b/includes/sanitizers/class-amp-style-sanitizer.php
index 783b98c26d7..28658d89112 100644
--- a/includes/sanitizers/class-amp-style-sanitizer.php
+++ b/includes/sanitizers/class-amp-style-sanitizer.php
@@ -1561,7 +1561,7 @@ private function get_parsed_stylesheet( $stylesheet, $options = [] ) {
 		$parsed         = null;
 		$cache_key      = null;
 		$cached         = true;
-		$cache_group    = 'amp-parsed-stylesheet-v35'; // This should be bumped whenever the PHP-CSS-Parser is updated or parsed format is updated.
+		$cache_group    = 'amp-parsed-stylesheet-v36'; // This should be bumped whenever the PHP-CSS-Parser is updated or parsed format is updated.
 		$use_transients = $this->should_use_transient_caching();
 
 		$cache_impacting_options = array_merge(
@@ -2157,24 +2157,7 @@ private function process_css_list( CSSList $css_list, $options ) {
 					$this->process_css_declaration_block( $css_item, $css_list, $options )
 				);
 			} elseif ( $css_item instanceof AtRuleBlockList ) {
-				if (
-					'-moz-document' === $css_item->atRuleName()
-					&&
-					'url-prefix()' === $css_item->atRuleArgs()
-					&&
-					in_array( 'supports', $options['allowed_at_rules'], true )
-				) {
-					// Replace `@-moz-document url-prefix()` with `@supports (-moz-appearance:meterbar)` as an alternative
-					// way to provide Firefox-specific style rules. This is a workaround since @-moz-document is not
-					// yet allowed in AMP, and this use of @supports is another recognized Firefox-specific CSS hack,
-					// per <http://browserhacks.com/#hack-8e9b5504d9fda44ec75169381b3c3157>.
-					// For adding @-moz-document to AMP, see <https://github.com/ampproject/amphtml/issues/26406>.
-					$new_css_item = new AtRuleBlockList( 'supports', '(-moz-appearance:meterbar)' );
-					$new_css_item->setContents( $css_item->getContents() );
-					$this->replace_inside_css_list( $css_list, $css_item, [ $new_css_item ] );
-					$css_item  = $new_css_item; // To process_css_list below.
-					$sanitized = false;
-				} elseif ( ! in_array( $css_item->atRuleName(), $options['allowed_at_rules'], true ) ) {
+				if ( ! in_array( $css_item->atRuleName(), $options['allowed_at_rules'], true ) ) {
 					$error                = [
 						'code'      => self::CSS_SYNTAX_INVALID_AT_RULE,
 						'at_rule'   => $css_item->atRuleName(),
diff --git a/tests/php/test-amp-style-sanitizer.php b/tests/php/test-amp-style-sanitizer.php
index 25a9f2ad78b..a84b8690779 100644
--- a/tests/php/test-amp-style-sanitizer.php
+++ b/tests/php/test-amp-style-sanitizer.php
@@ -206,11 +206,11 @@ public function get_body_style_attribute_data() {
 				'<style>@charset "UTF-8"; @charset "UTF-8"; @charset "UTF-8"; html:lang(zz){ color: gray; } @media screen and ( max-width: 640px ) { body { font-size: small; } } @font-face { font-family: "Open Sans"; src: url("/fonts/OpenSans-Regular-webfont.woff2") format("woff2"); } @-moz-document url-prefix() { body { color:red; } } @supports (display: grid) { div { display: grid; } } @-moz-keyframes appear { from { opacity: 0.0; } to { opacity: 1.0; } } @keyframes appear { from { opacity: 0.0; } to { opacity: 1.0; } }</style><div></div>',
 				'<div></div>',
 				[
-					'@media screen and ( max-width: 640px ){body{font-size:small}}@font-face{font-family:"Open Sans";src:url("/fonts/OpenSans-Regular-webfont.woff2") format("woff2")}@supports (-moz-appearance:meterbar){body{color:red}}@supports (display: grid){div{display:grid}}@-moz-keyframes appear{from{opacity:0}to{opacity:1}}@keyframes appear{from{opacity:0}to{opacity:1}}',
+					'@media screen and ( max-width: 640px ){body{font-size:small}}@font-face{font-family:"Open Sans";src:url("/fonts/OpenSans-Regular-webfont.woff2") format("woff2")}@-moz-document url-prefix(){body{color:red}}@supports (display: grid){div{display:grid}}@-moz-keyframes appear{from{opacity:0}to{opacity:1}}@keyframes appear{from{opacity:0}to{opacity:1}}',
 				],
 			],
 
-			'moz_document_transformed' => [
+			'moz_document_allowed' => [
 				'
 					<style>
 						@-moz-document url-prefix() {
@@ -228,7 +228,7 @@ public function get_body_style_attribute_data() {
 					<div class="entry"><div class="entry-content"><p class="has-drop-cap">Hello</p></div></div>
 				',
 				[
-					'@supports (-moz-appearance:meterbar){.entry .entry-content .has-drop-cap:not(:focus):first-letter{margin-top:.2em}}',
+					'@-moz-document url-prefix(){.entry .entry-content .has-drop-cap:not(:focus):first-letter{margin-top:.2em}}',
 				],
 				[
 					AMP_Style_Sanitizer::CSS_SYNTAX_INVALID_PROPERTY_NOLIST,