diff --git a/_locales/en/messages.json b/_locales/en/messages.json
index ea18706f..51dddb1a 100644
--- a/_locales/en/messages.json
+++ b/_locales/en/messages.json
@@ -468,6 +468,12 @@
     "message": "Import",
     "description": "Label for the button to import a style ('edit' page) or all styles ('manage' page)"
   },
+  "importPreprocessor": {
+    "message": "Style with a <code>@preprocessor</code> won't work in the classic mode. You can switch the editor to Usercss mode: 1) open the style manager, 2) enable \"as Usercss\" checkbox, 3) click \"Write new style\"\n\nImport now anyway?"
+  },
+  "importPreprocessorTitle": {
+    "message": "Potential problem due to @preprocessor"
+  },
   "importReplaceLabel": {
     "message": "Overwrite style",
     "description": "Label for the button to import and overwrite current style"
diff --git a/background/usercss-helper.js b/background/usercss-helper.js
index 00b3a99b..58dc2233 100644
--- a/background/usercss-helper.js
+++ b/background/usercss-helper.js
@@ -8,6 +8,7 @@ const usercssHelper = (() => {
   API_METHODS.configUsercssVars = configUsercssVars;
 
   API_METHODS.buildUsercss = build;
+  API_METHODS.buildUsercssMeta = buildMeta;
   API_METHODS.findUsercss = find;
 
   function buildMeta(style) {
diff --git a/edit/sections-editor-section.js b/edit/sections-editor-section.js
index 5b07875e..dce07e93 100644
--- a/edit/sections-editor-section.js
+++ b/edit/sections-editor-section.js
@@ -176,10 +176,9 @@ function createSection({
     });
     cm.on('paste', (cm, event) => {
       const text = event.clipboardData.getData('text') || '';
-      if (
-        text.includes('@-moz-document') &&
-        text.replace(/\/\*[\s\S]*?(?:\*\/|$)/g, '')
-          .match(/@-moz-document[\s\r\n]+(url|url-prefix|domain|regexp)\(/)
+      if (/@-moz-document/i.test(text) &&
+          /@-moz-document\s+(url|url-prefix|domain|regexp)\(/i
+            .test(text.replace(/\/\*([^*]|\*(?!\/))*(\*\/|$)/g, ''))
       ) {
         event.preventDefault();
         showMozillaFormatImport(text);
diff --git a/edit/sections-editor.js b/edit/sections-editor.js
index e2e3678d..567a5bc4 100644
--- a/edit/sections-editor.js
+++ b/edit/sections-editor.js
@@ -48,7 +48,7 @@ function createSectionsEditor(editorBase) {
 
   let sectionOrder = '';
   let headerOffset; // in compact mode the header is at the top so it reduces the available height
-  const ready = initSections(style.sections, {isFirstInit: true});
+  const ready = initSections(style.sections, {pristine: true});
 
   const livePreview = createLivePreview();
   livePreview.show(Boolean(style.id));
@@ -333,24 +333,37 @@ function createSectionsEditor(editorBase) {
       'Shift-Ctrl-Enter': () => doImport({replaceOldStyle: true}),
     };
 
-    function doImport({replaceOldStyle = false}) {
+    async function doImport({replaceOldStyle = false}) {
       lockPageUI(true);
-      API.parseCss({code: popup.codebox.getValue().trim()})
-        .then(({sections, errors}) => {
+      try {
+        const code = popup.codebox.getValue().trim();
+        if (!/==userstyle==/i.test(code) ||
+            !await getPreprocessor(code) ||
+            await messageBox.confirm(
+              t('importPreprocessor'), 'pre-line',
+              t('importPreprocessorTitle'))
+        ) {
+          const {sections, errors} = await API.parseCss({code});
           // shouldn't happen but just in case
           if (!sections.length || errors.length) {
             throw errors;
           }
-          if (replaceOldStyle) {
-            return replaceSections(sections);
-          }
-          return initSections(sections, {focusOn: false});
-        })
-        .then(() => {
+          await initSections(sections, {
+            replace: replaceOldStyle,
+            focusOn: replaceOldStyle ? 0 : false,
+          });
           $('.dismiss').dispatchEvent(new Event('click'));
-        })
-        .catch(showError)
-        .then(() => lockPageUI(false));
+        }
+      } catch (err) {
+        showError(err);
+      }
+      lockPageUI(false);
+    }
+
+    async function getPreprocessor(code) {
+      try {
+        return (await API.buildUsercssMeta({sourceCode: code})).usercssData.preprocessor;
+      } catch (e) {}
     }
 
     function lockPageUI(locked) {
@@ -451,8 +464,14 @@ function createSectionsEditor(editorBase) {
 
   function initSections(originalSections, {
     focusOn = 0,
-    isFirstInit,
+    replace = false,
+    pristine = false,
   } = {}) {
+    if (replace) {
+      sections.forEach(s => s.remove(true));
+      sections.length = 0;
+      container.textContent = '';
+    }
     let done;
     const total = originalSections.length;
     originalSections = originalSections.slice();
@@ -464,7 +483,7 @@ function createSectionsEditor(editorBase) {
       const t0 = performance.now();
       while (originalSections.length && performance.now() - t0 < 100) {
         insertSectionAfter(originalSections.shift(), undefined, forceRefresh);
-        if (isFirstInit) dirty.clear();
+        if (pristine) dirty.clear();
         if (focusOn !== false && sections[focusOn]) {
           sections[focusOn].cm.focus();
           focusOn = false;
@@ -572,36 +591,21 @@ function createSectionsEditor(editorBase) {
     updateSectionOrder();
   }
 
-  function replaceSections(...args) {
-    for (const section of sections) {
-      section.remove(true);
-    }
-    sections.length = 0;
-    container.textContent = '';
-    return initSections(...args);
-  }
-
-  function replaceStyle(newStyle, codeIsUpdated) {
+  async function replaceStyle(newStyle, codeIsUpdated) {
     dirty.clear('name');
     // FIXME: avoid recreating all editors?
-    reinit().then(() => {
-      Object.assign(style, newStyle);
-      updateHeader();
-      dirty.clear();
-      // Go from new style URL to edit style URL
-      if (location.href.indexOf('id=') === -1 && style.id) {
-        history.replaceState({}, document.title, 'edit.html?id=' + style.id);
-        $('#heading').textContent = t('editStyleHeading');
-      }
-      livePreview.show(Boolean(style.id));
-      updateLivePreview();
-    });
-
-    function reinit() {
-      if (codeIsUpdated !== false) {
-        return replaceSections(newStyle.sections, {isFirstInit: true});
-      }
-      return Promise.resolve();
+    if (codeIsUpdated !== false) {
+      await initSections(newStyle.sections, {replace: true, pristine: true});
     }
+    Object.assign(style, newStyle);
+    updateHeader();
+    dirty.clear();
+      // Go from new style URL to edit style URL
+    if (location.href.indexOf('id=') === -1 && style.id) {
+      history.replaceState({}, document.title, 'edit.html?id=' + style.id);
+      $('#heading').textContent = t('editStyleHeading');
+    }
+    livePreview.show(Boolean(style.id));
+    updateLivePreview();
   }
 }
diff --git a/msgbox/msgbox.css b/msgbox/msgbox.css
index bbc70622..4f349613 100644
--- a/msgbox/msgbox.css
+++ b/msgbox/msgbox.css
@@ -56,6 +56,10 @@
   text-align: left;
 }
 
+#message-box.pre-line #message-box-contents {
+  white-space: pre-line;
+}
+
 #message-box-title {
   font-weight: bold;
   background-color: rgb(145, 208, 198);
diff --git a/msgbox/msgbox.js b/msgbox/msgbox.js
index a537da32..6a409f9f 100644
--- a/msgbox/msgbox.js
+++ b/msgbox/msgbox.js
@@ -168,10 +168,12 @@ messageBox.alert = (contents, className, title) =>
 /**
  * @param {String|Node|Array<String|Node>} contents
  * @param {String} [className] like 'pre' for monospace font
+ * @param {String} [title]
  * @returns {Promise<Boolean>} resolves to true when confirmed
  */
-messageBox.confirm = (contents, className) =>
+messageBox.confirm = (contents, className, title) =>
   messageBox({
+    title,
     contents,
     className: `center ${className || ''}`,
     buttons: [t('confirmYes'), t('confirmNo')]