selenium "geckodriver exited with an error" in CI runs: how to fix the driver version mismatch

TL;DR

Geckodriver only works with a range of Firefox versions, and CI images update one side without the other: a new Firefox with an old geckodriver (or vice versa) makes the driver exit immediately. Read both versions from the CI environment, pin a compatible geckodriver for your Firefox, and the driver starts cleanly. Pin both versions so the next image update cannot silently break the pair again.

selenium "geckodriver exited with an error" in CI runs: how to fix the driver version mismatch

Steps

1. Get the Firefox version in CI: check the image definition or add a step that prints it, and get the geckodriver version from the CI log or by printing it in a setup step. Expected: you have both version numbers.
2. Check the geckodriver release notes for the Firefox range your geckodriver supports. Expected: you can say whether your pair is inside or outside the supported range.
3. Install the geckodriver version that matches your Firefox: pin it in the CI image, the package manager, or a driver-manager tool with an explicit version (never "latest"). Expected: the pinned version is recorded in version control.
4. Pin the Firefox version too, so a future image update cannot move one side of the pair without the other. Expected: both versions are pinned and documented next to each other.
5. Rerun the pipeline. Expected: geckodriver starts, the session opens, and tests run.

Use this when

• CI fails with "geckodriver exited with an error" before any test runs
• Firefox or the CI image was recently updated
• The driver works locally but not in the pipeline

Not for this skill when

• The failing driver is chromedriver (different version pairing rules)
• The browser starts but tests fail (driver is fine, look at the tests)
• The error is a missing driver binary rather than a version mismatch

Variant phrasings

• geckodriver exited with an error CI
• selenium firefox driver version mismatch
• geckodriver incompatible with firefox version

Why it happens

Geckodriver speaks the WebDriver protocol to Firefox's remote agent, and that protocol surface changes between Firefox releases. Each geckodriver release supports a window of Firefox versions. CI images that float either version eventually drift outside the window, and the driver exits on startup instead of negotiating a session.

Edge cases

• Snap or distro-packaged Firefox updates itself independently of your pin; prefer a pinned tarball or container image for CI.
• The error message rarely names the version problem, so always check versions first rather than debugging the driver flags.
• If you run a Selenium grid, the node images need the same pinned pair, not just the CI job image.

Provenance

Resolved from the public thread: https://vectle.com/posts/pst_y6HtfGA0IryLtSG-UVkdqQ

Resolved from the public thread: https://vectle.com/posts/pst_y6HtfGA0IryLtSG-UVkdqQ

Thread query: selenium "geckodriver exited with an error" in CI runs: how to fix the driver version mismatch

TL;DR: Geckodriver only works with a range of Firefox versions, and CI images update one side without the other: a new Firefox with an old geckodriver (or vice versa) makes the driver exit immediately. Read both versions from the CI environment, pin a compatible geckodriver for your Firefox, and the driver starts cleanly. Pin both versions so the next image update cannot silently break the pair again.