diff --git a/web/packages/extension/assets/css/common.css b/web/packages/extension/assets/css/common.css
index 8c3beb54e06f..2c370689bbc3 100644
--- a/web/packages/extension/assets/css/common.css
+++ b/web/packages/extension/assets/css/common.css
@@ -157,3 +157,19 @@ button {
     height: 20px;
     margin: auto;
 }
+
+/* Textarea input */
+.option.textarea {
+    flex-direction: column;
+}
+
+.option.textarea label {
+    color: var(--ruffle-orange);
+    font-size: 20px;
+    margin: 8px auto;
+    padding: 0;
+}
+
+.option.textarea textarea {
+    width: 100%;
+}
diff --git a/web/packages/extension/assets/options.html b/web/packages/extension/assets/options.html
index 6431a4121f44..51a7cec45611 100644
--- a/web/packages/extension/assets/options.html
+++ b/web/packages/extension/assets/options.html
@@ -66,6 +66,11 @@
                 <input type="number" id="max_execution_duration" min="1" />
                 <label for="max_execution_duration">Maximum allowed ActionScript execution duration (in seconds)</label>
             </div>
+            <div class="option textarea">
+                <label for="custom_config">Custom Ruffle config (JSON)</label>
+                <textarea id="custom_config" rows="10" placeholder='{"letterbox":"on"}'></textarea>
+                <button>Set custom config</button>
+            </div>
             <button id="reset-settings">Reset settings</button>
         </div>
         <script src="dist/options.js"></script>
diff --git a/web/packages/extension/src/common.ts b/web/packages/extension/src/common.ts
index 4dec1094742b..994e925fee9f 100644
--- a/web/packages/extension/src/common.ts
+++ b/web/packages/extension/src/common.ts
@@ -7,11 +7,13 @@ export interface Options extends Config.BaseLoadOptions {
     autostart: boolean;
     showReloadButton: boolean;
     swfTakeover: boolean;
+    customConfig?: string;
 }
 
 interface OptionElement<T> {
     readonly input: Element;
     readonly label: HTMLLabelElement;
+    readonly submitBtn?: HTMLButtonElement;
     value: T;
 }
 
@@ -110,6 +112,26 @@ class SelectOption implements OptionElement<string | null> {
     }
 }
 
+class TextareaOption implements OptionElement<string> {
+    constructor(
+        private readonly textarea: HTMLTextAreaElement,
+        readonly label: HTMLLabelElement,
+        readonly submitBtn: HTMLButtonElement,
+    ) {}
+
+    get input() {
+        return this.textarea;
+    }
+
+    get value() {
+        return this.textarea.value;
+    }
+
+    set value(value: string) {
+        this.textarea.value = value ?? "";
+    }
+}
+
 function getElement(option: Element): OptionElement<unknown> {
     const label = option.getElementsByTagName("label")[0]!;
 
@@ -129,6 +151,12 @@ function getElement(option: Element): OptionElement<unknown> {
         return new SelectOption(select, label);
     }
 
+    const [textarea] = option.getElementsByTagName("textarea");
+    if (textarea) {
+        const submitBtn = option.getElementsByTagName("button")[0]!;
+        return new TextareaOption(textarea, label, submitBtn);
+    }
+
     throw new Error("Unknown option element");
 }
 
@@ -168,12 +196,30 @@ export async function bindOptions(
             element.label.textContent = message;
         }
 
-        // Listen for user input.
-        element.input.addEventListener("change", () => {
-            const value = element.value;
-            options[key] = value as never;
-            utils.storage.sync.set({ [key]: value });
-        });
+        if (element.input.nodeName === "TEXTAREA" && element.submitBtn) {
+            element.submitBtn.addEventListener("click", () => {
+                const value = element.value as string;
+                if (value.trim() === "") {
+                    utils.storage.sync.set({ [key]: "" });
+                    alert("Custom configuration cleared.");
+                    return;
+                }
+                try {
+                    JSON.parse(value);
+                    utils.storage.sync.set({ [key]: value });
+                    alert("Custom configuration saved successfully.");
+                } catch (_e) {
+                    alert("Invalid configuration.");
+                }
+            });
+        } else {
+            // Listen for user input.
+            element.input.addEventListener("change", () => {
+                const value = element.value;
+                options[key] = value as never;
+                utils.storage.sync.set({ [key]: value });
+            });
+        }
     }
 
     // Listen for future changes.
diff --git a/web/packages/extension/src/content.ts b/web/packages/extension/src/content.ts
index a6ab01115ad6..eac7cbb76cf4 100644
--- a/web/packages/extension/src/content.ts
+++ b/web/packages/extension/src/content.ts
@@ -191,10 +191,10 @@ function isXMLDocument(): boolean {
     await sendMessageToPage({
         type: "load",
         config: {
-            ...explicitOptions,
             autoplay: options.autostart ? "on" : "auto",
             unmuteOverlay: options.autostart ? "hidden" : "visible",
             splashScreen: !options.autostart,
+            ...explicitOptions,
         },
         publicPath: utils.runtime.getURL("/dist/"),
     });
diff --git a/web/packages/extension/src/utils.ts b/web/packages/extension/src/utils.ts
index 35815cf2ff55..58f4ffffaad0 100644
--- a/web/packages/extension/src/utils.ts
+++ b/web/packages/extension/src/utils.ts
@@ -8,6 +8,7 @@ const DEFAULT_OPTIONS: Required<Options> = {
     autostart: false,
     showReloadButton: false,
     swfTakeover: true,
+    customConfig: "",
 };
 
 // TODO: Once https://crbug.com/798169 is addressed, just use browser.
@@ -106,6 +107,17 @@ export async function getExplicitOptions(): Promise<Options> {
         delete options["responseHeadersUnsupported"];
     }
 
+    // Handle customConfig JSON
+    if (options.customConfig) {
+        try {
+            const extra = JSON.parse(options.customConfig);
+            Object.assign(options, extra);
+        } catch (e) {
+            console.warn("Invalid custom_config JSON:", e);
+        }
+        delete options.customConfig;
+    }
+
     return options;
 }