Index: gc_lang/fr/webext/content_scripts/editor.js
==================================================================
--- gc_lang/fr/webext/content_scripts/editor.js
+++ gc_lang/fr/webext/content_scripts/editor.js
@@ -18,10 +18,13 @@
         //console.log(xDocument.body.innerHTML);
         this.lNode = [];
         this.bCheckSignature = bCheckSignature;
         this._lParsableNodes = ["P", "LI", "H1", "H2", "H3", "H4", "H5", "H6"];
         this._lRootNodes = ["DIV", "UL", "OL"];
+        if (bThunderbird) {
+            oGrammalecte.oGCPanel.addMessageToGCPanel("❗ Interface pour Thunderbird en version bêta.");
+        }
     }
 
     * _getParsableNodes (xRootNode) {
         // recursive function
         try {

Index: gc_lang/fr/webext/content_scripts/panel_gc.js
==================================================================
--- gc_lang/fr/webext/content_scripts/panel_gc.js
+++ gc_lang/fr/webext/content_scripts/panel_gc.js
@@ -60,10 +60,11 @@
         this.oTooltip = new GrammalecteTooltip(this.xParent, this.xGCPanelContent);
         this.xPanelContent.appendChild(this.xGCPanelContent);
         this.oTextControl = null;
         this.nLastResult = 0;
         this.iLastEditedParagraph = -1;
+        this.nParagraph = 0;
         // Lexicographer
         this.nLxgCount = 0;
         this.xLxgPanelContent = oGrammalecte.createNode("div", {id: "grammalecte_lxg_panel_content"});
         this.xPanelContent.appendChild(this.xLxgPanelContent);
         // Conjugueur
@@ -149,11 +150,11 @@
             // Text or node
             this.oTextControl = new TextNodeEditor(what, xResultNode);
         }
         else {
             // error
-            oGrammalecte.oMessageBox.showMessage("[BUG] Analyse d’un élément inconnu…");
+            oGrammalecte.showMessage("[BUG] Analyse d’un élément inconnu…");
             console.log("[Grammalecte] Unknown element:", what);
         }
     }
 
     setAutoRefreshButton () {
@@ -214,10 +215,14 @@
         }
         this.aIgnoredErrors.clear();
     }
 
     hide () {
+        if (bThunderbird) {
+            oGrammalecte.showMessage("Veuillez patienter…");
+            this.copyAllParagraphsToComposeWindow();
+        }
         if (oGrammalecte.oTFPanel) { oGrammalecte.oTFPanel.hide(); }
         if (oGrammalecte.oMessageBox) { oGrammalecte.oMessageBox.hide(); }
         oGrammalecte.clearRightClickedNode();
         this.xPanel.style.display = "none";
         this.oTextControl.clear();
@@ -250,10 +255,11 @@
                 this._tagParagraph(xParagraph, oResult.sParagraph, oResult.iParaNum, oResult.aGrammErr, oResult.aSpellErr);
                 // creation
                 xNodeDiv.appendChild(xActionsBar);
                 xNodeDiv.appendChild(xParagraph);
                 this.xParagraphList.appendChild(xNodeDiv);
+                this.nParagraph += 1;
             }
         }
         catch (e) {
             showError(e);
         }
@@ -441,10 +447,21 @@
     }
 
     addSummary () {
         // todo
     }
+
+    copyAllParagraphsToComposeWindow () {
+        // Thunderbird only
+        // When closing the window, we change all nodes according to the content of paragraphs in the gc panel
+        for (let iPara = 0;  iPara < this.nParagraph;  iPara++) {
+            let sParagraphId = "grammalecte_paragraph"+iPara;
+            if (this.xParent.getElementById(sParagraphId)) {
+                this.oTextControl.setParagraph(iPara, this.xParent.getElementById(sParagraphId).textContent);
+            }
+        }
+    }
 
     addMessageToGCPanel (sMessage) {
         let xNode = oGrammalecte.createNode("div", {className: "grammalecte_panel_flow_message", textContent: sMessage});
         this.xParagraphList.appendChild(xNode);
     }