Fix Firefox cookie expiry conversion for Playwright

Some Firefox cookies store expiry in milliseconds instead of seconds.
Detect values beyond year 2100 and convert to seconds. Also map
zero/negative expiry to -1 for Playwright session cookies.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

dist/firefox.js

@@ -215,12 +215,22 @@ export function importFirefoxCookies(options) {
  * Convert FirefoxCookie to Playwright cookie format
  */
 export function toPlaywrightCookie(cookie) {
+    // Firefox uses 0 for session cookies; Playwright requires -1 or positive unix timestamp (seconds).
+    // Some Firefox cookies store expiry in milliseconds instead of seconds — detect and convert.
+    // Any expiry > year 2100 in seconds (4102444800) is likely milliseconds.
+    let expires = cookie.expires;
+    if (expires > 4102444800) {
+        expires = Math.floor(expires / 1000);
+    }
+    if (expires <= 0) {
+        expires = -1;
+    }
     return {
         name: cookie.name,
         value: cookie.value,
         domain: cookie.domain,
         path: cookie.path,
-        expires: cookie.expires,
+        expires,
         secure: cookie.secure,
         httpOnly: cookie.httpOnly,
         sameSite: cookie.sameSite,