Fix Firefox “already running” or profile-in-use errors
What to do first
Troubleshoot Firefox “already running” or profile-in-use errors by following the official decision path while preserving user data.
This is a general troubleshooting guide, not an announcement of a current outage.
Affected products and symptoms
- Product
- Mozilla Firefox
- Versions and conditions
- Firefox Desktop
- Error codes and identifiers
- Firefox is already running but is not responding · parent.lock
Symptoms
- Firefox says it is already running after exit
- A profile-in-use error blocks startup
Causes and conditions
Check remaining Firefox processes, stale lock files, and shared-folder permission.
Before you start
- Before changes, record and preserve the entire Firefox profile and whether open tabs can be restored.
- Change one item at a time and record the original symptom after each stage.
Precautions
- Do not bulk-delete unrelated settings or data.
- Exclude personal data and credentials; share only the necessary parts of the entire Firefox profile and whether open tabs can be restored.
Step-by-step instructions
Open Task Manager with Ctrl+Shift+Esc and End task on Firefox.
Unsaved form content can be lost.
End task interrupts Firefox writes. Wait briefly and confirm the process disappears before touching the profile.
Restart Windows and launch Firefox once.
If recurring, inspect the profile location.
If the location is unknown, open %APPDATA%\Mozilla\Firefox\Profiles and compare candidates with profiles.ini.
With Firefox fully closed, move parent.lock out of the profile.
Never remove it while Firefox is actually running.
parent.lock prevents concurrent profile use. Move it aside only after Firefox is truly stopped, preserving rollback.
Connect a file share first and confirm read/write permission.
If still locked, create a new profile and migrate old data.
On NAS or synchronized storage, disconnects and read-only access can recur. Compare with a local profile.
Check the result
- start and exit Firefox normally, then relaunch the same profile without another lock
- Check adjacent basic functions for side effects from the change.
If the problem continues
- start a new profile and migrate bookmarks and other data using Mozilla guidance
- When continuing with a workaround, preserve original files or account data separately.
Scope of this guide
Troubleshooting guide — Troubleshoot Firefox “already running” or profile-in-use errors by following the official decision path while preserving user data.
Frequently asked questions
Will bookmarks be deleted?
Moving the lock does not delete them, but back up the profile.
Why every time?
Investigate shutdown hangs, shared storage, and permissions.
Official sources and dates
Source publication or resolution date: Not specified. Sources checked: 2026-09-05. The check date is not the date the problem first occurred. Interface labels can vary between versions and display languages.
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