Firefox failing to start with Firefox 141 and pre-Cypress 14

[wpuckett-sd]

Current behavior

Have tried a few versions of cypress throughout the 13 release and each time I (or others) try to run a test in firefox we receive:
Failed to spawn CDP with Firefox. Retrying...
Failed to spawn CDP with Firefox. Retrying again...
The browser never connected. Something is wrong. The tests cannot run. Aborting...
The browser never connected. Something is wrong. The tests cannot run. Aborting...

This was working fine as of July 31 2025, as I've have tests running in firefox daily for a couple years now. Issue only started occurring today

I installed 14.5.3 and the issue doesn't seem to occur there, so as a workaround I'll try and fix up the upgrade issues i've experienced when moving up to version 14 before

Desired behavior

Launch firefox without issue

Test code to reproduce

npx cypress run --spec yourspecdirectoryhere --browser firefox

Cypress Version

13.17.0

Node version

22.17.0

Operating System

windows 10.0.26100

Debug Logs

Other

No response

[MikeMcC399]

@wpuckett-sd

Firefox removed Chrome DevTools Protocol (CDP) in Firefox 141 after deprecating it in Firefox 129. (See https://fxdx.dev/cdp-retirement-in-firefox/)

Cypress 14.1.0 added support for WebDriver BiDi for Firefox 135 and above.

So you would need a minimum of Cypress 14.1.0 to test against Firefox 141.

This information should probably be added to the documentation https://docs.cypress.io/app/references/launching-browsers#Firefox-Browsers

There are still some unsolved issues on Windows however:

• "[firefox-profile] cannot delete profileDir on exit" on Windows #31324

[MikeMcC399]

Firefox https://bugzilla.mozilla.org/show_bug.cgi?id=1882096 notes

Thanks for asking! I don't think that we have to mention the removal of our experimental CDP implementation in the official release notes of Firefox. It's only affecting developers and testers and as such is better suited to be added to the release notes on MDN, which we will take care of.

however it doesn't seem to be mentioned in https://developer.mozilla.org/en-US/docs/Mozilla/Firefox/Releases/141 so far.

[wpuckett-sd]

Gotcha, thanks @MikeMcC399. Obviously nothing Cypress can do about the CDP removal but upgrading to 14+ leads to many issues in our scripts (especially in firefox) so I'll give running against older versions of firefox a try before fully committing to fixing the v14 issues that pop up for us

[MikeMcC399]

@whimboo

Sorry for pinging! Since https://bugzilla.mozilla.org/show_bug.cgi?id=1882096 is closed, I can't comment there. Can you confirm that CDP has been removed in Firefox 141 stable? Is there also a release note that documents this?

[MikeMcC399]

It was however announced in https://fxdx.dev/webdriver-bidi-becomes-the-default-for-cypress-in-firefox/

WebDriver BiDi Becomes the Default for Cypress with Firefox

Cypress will use WebDriver BiDi as its default automation protocol for Firefox 135 and later, starting with Cypress 14.1.0.

CDP Support Ends with Firefox 141

As we continue transitioning to WebDriver BiDi, support for the experimental Chrome DevTools Protocol in Firefox will officially end with Firefox 141, set for release on July 22nd. The 140 ESR release will continue to have CDP support for its full support lifecycle. Removing the CDP implementation will allow us to focus our efforts entirely on improving WebDriver BiDi.

[MikeMcC399]

• See also documentation suggestion Add browser support section cypress-documentation#6237

[jennifer-shehane]

Sorry this was not clear in the documentation. We were intending to have this much clearer in the Cypress 15 documentation, but it's not released yet.

As Mike mentioned, you'll need a minimum of Cypress 14.1.0 to test against Firefox 141.

I'll close this issue as there's nothing we can do or plan to do to make the previous Cypress versions compatible with Firefox 141+. We got the Webdriver support into the earliest version of Cypress that we possibly could.

[MikeMcC399]

The requirement for Firefox 141 is now documented on https://docs.cypress.io/app/get-started/install-cypress#Browsers

[whimboo]

however it doesn't seem to be mentioned in https://developer.mozilla.org/en-US/docs/Mozilla/Firefox/Releases/141 so far.

Good find! We are going to update the release notes via mdn/content#40705.

[MikeMcC399]

@whimboo

however it doesn't seem to be mentioned in https://developer.mozilla.org/en-US/docs/Mozilla/Firefox/Releases/141 so far.

Good find! We are going to update the release notes via mdn/content#40705.

Thank you! It will be good to have that published and confirmed.