From MozillaZine Knowledge Base
If you are using multiple Firefox profiles, these tips may be helpful:
General tips for multiple profiles
- When you have multiple Firefox profiles, you can select the profile of your choice when starting Firefox by unchecking the Profile Manager option, "Don't ask at startup". This will always start the Profile Manager when starting Firefox.
- You can use a Firefox shortcut or command line argument to start Firefox with a specified profile without going through the Profile Manager.
- If you check the "Don't ask at startup" Profile Manager option when selecting a profile and start Firefox, that profile becomes the "default" profile and will be used automatically when starting Firefox (unless you use a shortcut or command line argument to start with a specified profile).
- Note: The term "default profile" should not be confused with "default browser". "Default browser" is a system setting which determines which web browser is launched when you open an Internet shortcut, HTML file, or link in a mail application like Thunderbird or Outlook Express, when more than one browser is installed on your system. The "default profile" is the last profile selected in the Profile Manager when starting Firefox and is used automatically if the "Don't ask at startup" Profile Manager option is checked.
- Keep in mind that profile names are case-sensitive so you may wish to avoid capitalizing profile names and use all lower-case.
- Even though you can have spaces in a profile name it is not a good idea. Profile names with spaces must be enclosed in quotes when used in a command line argument or in a shortcut target. There are instances where you need to specify a profile name but cannot enclose the profile name in quotes, such as in launchy.xml used by by the "Launchy" extension.
- If you specify a missing or incorrect profile name or path in a Firefox shortcut or command line argument, you may see the Profile Manager (using
-P "<profile name>") or a Firefox is already running but is not responding error (using
-profile "<path>"). Remember that the profile name is case sensitive and you must use quotes when there are spaces. See also Profile in use and Recovering a missing profile.
- Avoid renaming a profile in the Profile Manager as it does not change the actual profile folder name in the file system, even though it is changed in the profiles.ini file, which may lead to confusion.
- In Firefox 3.6 and above, you can use about:support (directly) or through "Help > Troubleshooting Information > Profile Directory: Open Containing Folder" (Windows and Linux) or "Show in Finder" (Mac OS) to open the profile folder for the profile currently in use. Prior to Firefox 3.6 suggest installing the Open Profile Folder extension, at least in your default profile, to give you quick access to your Firefox profile data.
- Use Color Toolbar(s) to identify profile | userstyles.org styling to color code your Firefox browser and dialog windows to identify the profile in use. The style is installed, used and managed with the Stylish extension available at addons. Also to help identify the profile create a bookmark folder named to identify the profile or the Firefox version that will be using the profile so you can see it when extensions and/or styling is disabled, also eliminates the need to use an extension to show the information in the title bar.
- (Windows) Create Windows Quick Launch shortcuts
- (Windows) Use the Launchy extension to open current page in another profile with a customized "launchy.xml" file (sample) with -no-remote to start each profile. (Don't use it to launch your default profile, at least not with -no-remote).  .
- Use a JSON bookmarkbackups file from your default profile to restore to your test profiles, if you need bookmarks, bookmarklets, or keyword shortcuts (details here). (Note: Bookmark backups in JSON format were introduced in Firefox 3)
Using new profiles in secondary Firefox installations
In addition to the major points have already been mentioned above:
- Use the Profile Manager to create a new profile for each secondary Firefox installation, such as a Beta build or another version of Firefox. To prevent accidental launching of that profile with another Firefox installation, uncheck the "Don't ask at startup" option in the Profile Manager.
- Start the secondary Firefox installation with a Firefox shortcut or command line argument that points to the profile you want to use with that Firefox installation. See Starting your Mozilla application with a specified profile for more information.
- When you first start a secondary installation of Firefox (e.g., a Beta or other test build) with a newly-created profile, always reply "Do not make Firefox your default browser" if you want to keep your primary installation of Firefox as the default browser.
- (Windows) Create a new shortcut icon for a specific Firefox version by right clicking on the startup icon, then under properties, shortcut, change icon. Won't apply to a specific profile but will help identify the version of Firefox.
Installing a pre-release version of Firefox
Beta versions will install the Firefox application into their own directories but, on Windows, always use custom install anyway (see Installing Firefox on Windows for details), as you would need to do if testing another released version of Firefox.
- You will want to create a new profile to be used only with your Beta or other version of Firefox.
- Create the profile with the Profile Manager using the version of Firefox you intend to use with the profile.
- Do not make Firefox your default browser when using this new profile.
- Don't allow installation to start Firefox for you, as you need to create the profile, and need to modify the desktop shortcut to include the profile name and the addition of -no-remote to the start-up shortcut, along with renaming the shortcut and possibly a change of icon.
- Modify shortcut to include both name of profile and -no-remote as it will not be your default browser/profile
- Start up Firefox with the new shortcut to create the new profile, you will have to name the new profile, Firefox will just recognize that it doesn't have the profile you asked for (profiles are case sensitive) but don't let Firefox start as it may not pick up -no-remote
- Open the new Firefox and/or profile with your new shortcut.
Release Candidates are normally installed as a replacement for your existing Firefox application and, by default, will be installed into the default Firefox installation directory location.
See Testing pre-release versions for more information.
Windows Task Manager
Additional columns can be added within the Processes tab of the Windows Task Manager (WTM) by right-clicking on a column header. When working with multiple versions of Firefox at the same time, you will need to know which process to terminate if there is a problem. By including the "Command Line" column you will see the actual command line that was invoked to start each process. Typical problem would be a stuck application not showing on the Windows status bar but still in the WTM Processes tab. Trying to start/restart Firefox in such cases will show a dialog box with the message, Firefox is already running but is not responding. To open a new window, you must first close the existing Firefox process, or restart the system.