diff --git a/dist/firefox.d.ts.map b/dist/firefox.d.ts.map
index 8df0805..f931ba7 100644
--- a/dist/firefox.d.ts.map
+++ b/dist/firefox.d.ts.map
@@ -1 +1 @@
-{"version":3,"file":"firefox.d.ts","sourceRoot":"","sources":["../src/firefox.ts"],"names":[],"mappings":"AAAA;;;;;;;;;;;;;;;GAeG;AAOH,MAAM,WAAW,aAAa;IAC5B,IAAI,EAAE,MAAM,CAAC;IACb,KAAK,EAAE,MAAM,CAAC;IACd,MAAM,EAAE,MAAM,CAAC;IACf,IAAI,EAAE,MAAM,CAAC;IACb,OAAO,EAAE,MAAM,CAAC;IAChB,MAAM,EAAE,OAAO,CAAC;IAChB,QAAQ,EAAE,OAAO,CAAC;IAClB,QAAQ,EAAE,MAAM,GAAG,KAAK,GAAG,QAAQ,CAAC;CACrC;AAED,UAAU,cAAc;IACtB,IAAI,EAAE,MAAM,CAAC;IACb,IAAI,EAAE,MAAM,CAAC;IACb,UAAU,EAAE,OAAO,CAAC;IACpB,SAAS,EAAE,OAAO,CAAC;CACpB;AAuFD;;GAEG;AACH,wBAAgB,mBAAmB,IAAI,cAAc,EAAE,CAItD;AAsFD;;GAEG;AACH,wBAAgB,oBAAoB,CAAC,OAAO,CAAC,EAAE;IAC7C,OAAO,CAAC,EAAE,MAAM,CAAC;IACjB,MAAM,CAAC,EAAE,MAAM,CAAC;CACjB,GAAG,aAAa,EAAE,CAmClB;AAED;;GAEG;AACH,wBAAgB,kBAAkB,CAAC,MAAM,EAAE,aAAa,GAAG;IACzD,IAAI,EAAE,MAAM,CAAC;IACb,KAAK,EAAE,MAAM,CAAC;IACd,MAAM,EAAE,MAAM,CAAC;IACf,IAAI,EAAE,MAAM,CAAC;IACb,OAAO,EAAE,MAAM,CAAC;IAChB,MAAM,EAAE,OAAO,CAAC;IAChB,QAAQ,EAAE,OAAO,CAAC;IAClB,QAAQ,EAAE,QAAQ,GAAG,KAAK,GAAG,MAAM,CAAC;CACrC,CAWA"}
\ No newline at end of file
+{"version":3,"file":"firefox.d.ts","sourceRoot":"","sources":["../src/firefox.ts"],"names":[],"mappings":"AAAA;;;;;;;;;;;;;;;GAeG;AAOH,MAAM,WAAW,aAAa;IAC5B,IAAI,EAAE,MAAM,CAAC;IACb,KAAK,EAAE,MAAM,CAAC;IACd,MAAM,EAAE,MAAM,CAAC;IACf,IAAI,EAAE,MAAM,CAAC;IACb,OAAO,EAAE,MAAM,CAAC;IAChB,MAAM,EAAE,OAAO,CAAC;IAChB,QAAQ,EAAE,OAAO,CAAC;IAClB,QAAQ,EAAE,MAAM,GAAG,KAAK,GAAG,QAAQ,CAAC;CACrC;AAED,UAAU,cAAc;IACtB,IAAI,EAAE,MAAM,CAAC;IACb,IAAI,EAAE,MAAM,CAAC;IACb,UAAU,EAAE,OAAO,CAAC;IACpB,SAAS,EAAE,OAAO,CAAC;CACpB;AAuFD;;GAEG;AACH,wBAAgB,mBAAmB,IAAI,cAAc,EAAE,CAItD;AAsFD;;GAEG;AACH,wBAAgB,oBAAoB,CAAC,OAAO,CAAC,EAAE;IAC7C,OAAO,CAAC,EAAE,MAAM,CAAC;IACjB,MAAM,CAAC,EAAE,MAAM,CAAC;CACjB,GAAG,aAAa,EAAE,CAmClB;AAED;;GAEG;AACH,wBAAgB,kBAAkB,CAAC,MAAM,EAAE,aAAa,GAAG;IACzD,IAAI,EAAE,MAAM,CAAC;IACb,KAAK,EAAE,MAAM,CAAC;IACd,MAAM,EAAE,MAAM,CAAC;IACf,IAAI,EAAE,MAAM,CAAC;IACb,OAAO,EAAE,MAAM,CAAC;IAChB,MAAM,EAAE,OAAO,CAAC;IAChB,QAAQ,EAAE,OAAO,CAAC;IAClB,QAAQ,EAAE,QAAQ,GAAG,KAAK,GAAG,MAAM,CAAC;CACrC,CAsBA"}
\ No newline at end of file
diff --git a/dist/firefox.js b/dist/firefox.js
index 230c2d8..8ded302 100644
--- a/dist/firefox.js
+++ b/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,
diff --git a/dist/firefox.js.map b/dist/firefox.js.map
index 0f407be..a3b8e59 100644
--- a/dist/firefox.js.map
+++ b/dist/firefox.js.map
@@ -1 +1 @@
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\ No newline at end of file
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\ No newline at end of file
diff --git a/src/firefox.ts b/src/firefox.ts
index cfd2248..553d5ff 100644
--- a/src/firefox.ts
+++ b/src/firefox.ts
@@ -272,12 +272,23 @@ export function toPlaywrightCookie(cookie: FirefoxCookie): {
   httpOnly: boolean;
   sameSite: 'Strict' | 'Lax' | 'None';
 } {
+  // 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,