Firefox profiles let you run completely separate browser instances — each with its own history, bookmarks, cookies, extensions, and settings. Here's how to use them effectively.
What Profiles Are For
A profile is a complete, isolated Firefox user configuration. Each profile maintains:
- Bookmarks and history
- Saved passwords
- Cookies and site data
- Extensions
- Settings (about:config, preferences)
- Themes
Common use cases:
- Work vs. personal: separate Google accounts, separate browsing history
- Testing: a clean profile for testing websites without your normal extensions interfering
- Developer testing: reproduce user issues in a fresh profile
- Privacy-focused profile: hardened settings, no history, separate from daily use
- Shared computers: each person gets their own profile
Creating and Managing Profiles
Via the Profile Manager
about:profiles
Or launch Firefox with --ProfileManager flag:
firefox --ProfileManager
The Profile Manager lets you:
- Create new profiles (with name and optional directory)
- Rename profiles
- Delete profiles
- Set the default profile
- Launch a specific profile
Running Multiple Profiles Simultaneously
By default, Firefox prevents running two instances of the same Firefox install. To run two profiles simultaneously, you need --no-remote:
# Start second profile
firefox -P "work-profile" --no-remote
Or use the profile manager to set up different Firefox installations pointing at different profiles.
On macOS: Multiple Firefox.app instances can run simultaneously if they use different profiles.
Alternative: Firefox Developer Edition has its own separate profile and can run alongside regular Firefox without conflict.
Profile Directory Structure
Each profile directory contains:
[profile-name]/
├── prefs.js # User preferences (modified from about:config)
├── user.js # Custom startup preferences (manually created)
├── places.sqlite # Bookmarks and history database
├── key4.db # Saved passwords (encrypted)
├── cookies.sqlite # Cookies
├── extensions/ # Installed extension files
├── chrome/ # userChrome.css, userContent.css
├── cache2/ # HTTP cache (large, can be moved)
├── sessionstore*.js # Session/tab state
└── ...
The profile directory is portable — you can copy it to another machine and Firefox will have the same state (minus cached content).
Using Profiles for Development
Clean test profile: Keep a "test" profile with no extensions, default settings, and cleared history. Use it to test:
- How your website looks without uBlock Origin
- How your site performs for a new user (no cached content)
- Whether a bug is extension-related (extensions are isolated per profile)
Extension testing: Test extensions in an isolated profile to prevent your normal extensions from interfering.
Multiple Firefox versions: You can have Firefox Stable and Firefox Nightly each use different profiles for completely separate testing environments.
Backing Up Profiles
Backup the profile folder — copy the entire directory. This preserves everything.
Key files to back up specifically:
-
places.sqlite— bookmarks and history -
key4.db+logins.json— saved passwords -
prefs.js— preferences -
chrome/folder — userChrome.css customizations
Firefox Sync is another backup mechanism — when enabled, bookmarks, passwords, and history sync to Mozilla's servers. But it's partial (doesn't sync extensions or about:config preferences).
Profile-Specific user.js
Each profile can have its own user.js file for startup preferences. This lets you maintain different privacy/performance profiles:
Production profile (user.js):
user_pref("browser.tabs.tabMinWidth", 50);
user_pref("privacy.resistFingerprinting", false); // Keep compatibility
Hardened profile (user.js):
user_pref("privacy.firstparty.isolate", true);
user_pref("privacy.resistFingerprinting", true);
user_pref("media.peerconnection.enabled", false);
Profile Cleanup
Old profiles accumulate:
- Cache data (can be several GB)
- History (can be millions of entries)
If a profile is old and unused:
- Launch Profile Manager (
about:profiles) - Click "Open Folder" on the profile to check its size
- Delete if no longer needed
Note: deleting a profile from the Profile Manager removes it from Firefox's list but may not delete the actual files — you may need to manually delete the profile folder from disk.
For most users, one profile for personal and one for work is a meaningful improvement over mixing contexts. The about:profiles page is a much better interface than most people expect — worth visiting once to see your current profiles and clean up any old ones.
