Bug 2062055 - [firefox-devtools-mcp] resolve geckodriver on all platforms so aarch64 Linux can launch

src/firefox/core.ts

@@ -349,18 +349,17 @@ export class FirefoxCore {
         }
       }
 
-      let serviceBuilder;
-      if (process.platform === 'win32') {
-        // On windows, firefox.ServiceBuilder() invoked from the MCP will hang.
-        // geckodriver has to be in the PATH. See Bug 2040849.
-        const geckodriverPath = await findGeckodriver();
-        logDebug(`Using geckodriver: ${geckodriverPath}`);
-        serviceBuilder = new firefox.ServiceBuilder(geckodriverPath);
-      } else {
-        // On other platforms, the default ServiceBuilder should locate and
-        // start geckodriver successfully.
-        serviceBuilder = new firefox.ServiceBuilder();
-      }
+      // Always resolve geckodriver ourselves rather than letting selenium-webdriver
+      // fall back to its bundled selenium-manager. The Linux selenium-manager
+      // shipped with selenium-webdriver 4.36.0 is an x86-64 binary, so on an
+      // aarch64 host it fails with "Exec format error" and the session dies with
+      // "Unable to obtain browser driver" even when a native geckodriver is in
+      // PATH. See Bug 2062055. On Windows the same call would hang instead
+      // (Bug 2040849). findGeckodriver() checks PATH, then the selenium cache,
+      // then downloads a platform-correct binary via the geckodriver package.
+      const geckodriverPath = await findGeckodriver();
+      logDebug(`Using geckodriver: ${geckodriverPath}`);
+      const serviceBuilder = new firefox.ServiceBuilder(geckodriverPath);
 
       if (this.logFilePath) {
         // Open file for appending, create if doesn't exist

tests/firefox/core.test.ts

@@ -317,6 +317,7 @@ describe('FirefoxCore connect() profile handling', () => {
   const mockWindowSize = vi.fn();
   const mockSetAcceptInsecureCerts = vi.fn();
   const mockSetStdio = vi.fn();
+  const mockServiceBuilderCtor = vi.fn();
 
   beforeEach(() => {
     vi.clearAllMocks();
@@ -333,7 +334,11 @@ describe('FirefoxCore connect() profile handling', () => {
           setAcceptInsecureCerts = mockSetAcceptInsecureCerts;
         },
         ServiceBuilder: class {
+          constructor(...args: unknown[]) {
+            mockServiceBuilderCtor(...args);
+          }
           setStdio = mockSetStdio;
+          addArguments = vi.fn();
         },
       },
     }));
@@ -383,4 +388,20 @@ describe('FirefoxCore connect() profile handling', () => {
       join('/path/to/test/profile', MCP_PROFILE_DIR_NAME)
     );
   });
+
+  // Bug 2062055: with no geckodriver path, selenium-webdriver falls back to its
+  // bundled selenium-manager, whose Linux binary is x86-64 only — so on aarch64
+  // the session dies with "Unable to obtain browser driver" even when a native
+  // geckodriver is in PATH. Resolving the path ourselves avoids that entirely.
+  it('should build the geckodriver service with an explicit binary path', async () => {
+    const { FirefoxCore } = await import('@/firefox/core.js');
+
+    const core = new FirefoxCore({ headless: true });
+    await core.connect();
+
+    expect(mockServiceBuilderCtor).toHaveBeenCalledTimes(1);
+    const [geckodriverPath] = mockServiceBuilderCtor.mock.calls[0] as [unknown];
+    expect(typeof geckodriverPath).toBe('string');
+    expect(String(geckodriverPath)).toContain('geckodriver');
+  });
 });