diff --git a/src/vs/base/browser/domStylesheets.ts b/src/vs/base/browser/domStylesheets.ts
index 76a71c5402dd0..feb3aeba8d7cd 100644
--- a/src/vs/base/browser/domStylesheets.ts
+++ b/src/vs/base/browser/domStylesheets.ts
@@ -5,6 +5,7 @@
 
 import { DisposableStore, toDisposable, IDisposable } from '../common/lifecycle.js';
 import { autorun, IObservable } from '../common/observable.js';
+import { isFirefox } from './browser.js';
 import { getWindows, sharedMutationObserver } from './dom.js';
 import { mainWindow } from './window.js';
 
@@ -97,7 +98,15 @@ function cloneGlobalStyleSheet(globalStylesheet: HTMLStyleElement, globalStylesh
 		clone.sheet?.insertRule(rule.cssText, clone.sheet?.cssRules.length);
 	}
 
-	disposables.add(sharedMutationObserver.observe(globalStylesheet, disposables, { childList: true })(() => {
+	let observeInit: MutationObserverInit = { childList: true };
+	if (isFirefox) {
+		// Firefox doesn't support observing style tag contents
+		// As a workaround, also observe the data-version attribute
+		// that is updated when the content is updated
+		observeInit = { ...observeInit, attributes: true, attributeFilter: ['data-version'] };
+	}
+
+	disposables.add(sharedMutationObserver.observe(globalStylesheet, disposables, observeInit)(() => {
 		clone.textContent = globalStylesheet.textContent;
 	}));
 
diff --git a/src/vs/workbench/services/themes/browser/workbenchThemeService.ts b/src/vs/workbench/services/themes/browser/workbenchThemeService.ts
index d85b23015f45c..972e265195504 100644
--- a/src/vs/workbench/services/themes/browser/workbenchThemeService.ts
+++ b/src/vs/workbench/services/themes/browser/workbenchThemeService.ts
@@ -33,7 +33,7 @@ import { updateColorThemeConfigurationSchemas, updateFileIconThemeConfigurationS
 import { ProductIconThemeData, DEFAULT_PRODUCT_ICON_THEME_ID } from './productIconThemeData.js';
 import { registerProductIconThemeSchemas } from '../common/productIconThemeSchema.js';
 import { ILogService } from '../../../../platform/log/common/log.js';
-import { isWeb } from '../../../../base/common/platform.js';
+import { isFirefox, isWeb } from '../../../../base/common/platform.js';
 import { ColorScheme, ThemeTypeSelector } from '../../../../platform/theme/common/theme.js';
 import { IHostColorSchemeService } from '../common/hostColorSchemeService.js';
 import { RunOnceScheduler, Sequencer } from '../../../../base/common/async.js';
@@ -42,6 +42,7 @@ import { getIconsStyleSheet } from '../../../../platform/theme/browser/iconsStyl
 import { asCssVariableName, getColorRegistry } from '../../../../platform/theme/common/colorRegistry.js';
 import { ILanguageService } from '../../../../editor/common/languages/language.js';
 import { mainWindow } from '../../../../base/browser/window.js';
+import { generateUuid } from '../../../../base/common/uuid.js';
 
 // implementation
 
@@ -793,13 +794,21 @@ class ThemeFileWatcher {
 }
 
 function _applyRules(styleSheetContent: string, rulesClassName: string) {
-	const themeStyles = mainWindow.document.head.getElementsByClassName(rulesClassName);
-	if (themeStyles.length === 0) {
-		const elStyle = createStyleSheet();
-		elStyle.className = rulesClassName;
-		elStyle.textContent = styleSheetContent;
-	} else {
-		(<HTMLStyleElement>themeStyles[0]).textContent = styleSheetContent;
+	let themeStyle = mainWindow.document.head.getElementsByClassName(rulesClassName).item(0);
+	if (!themeStyle) {
+		themeStyle = createStyleSheet();
+		themeStyle.className = rulesClassName;
+	}
+
+	if (themeStyle.textContent !== styleSheetContent) {
+		themeStyle.textContent = styleSheetContent;
+
+		if (isFirefox) {
+			// Firefox doesn't support observing style tag contents
+			// As a workaround, also update the data-version attribute
+			// when it changes so it can be observed
+			themeStyle.setAttribute('data-version', generateUuid());
+		}
 	}
 }