Recently Thunderbird started opening http URLs in the wrong browser. Although you may think that the solution would be a simple configuration change, as I did at the time, it turns out that the process which Thunderbird uses to determine which browser to use is complex, poorly documented, and has changed several times between Thunderbird versions. This post outlines my understanding of the process and, most importantly, how to change the default browser in current versions of Thunderbird.

How Thunderbird Chooses a Browser

Thunderbird has a variety of methods available for determining which browser to use. The methods are attempted one at a time until a browser is found.

Update: Raman Gupta pointed out that my original determination of the order in which the methods are attempted was incorrect and after further testing it appears that mimeTypes.rdf is consulted before XDG MIME action (at least on Thunderbird 10 and later, probably on previous versions as well).

This realization also prompted me to dig into the Thunderbird sources to get a better idea of how the process works. The specific order in which the methods are tried, and which methods are available, is based on the platform and the compilation options used for the running version of Thunderbird. Interested developers can look at GetProtocolHandler in nsExternalHelperAppService for the starting point, GetProtocolHandlerInfoFromOS in subclasses of nsExternalHelperAppService for the platform-specific bits and the HandlerService implementation for the platform-agnostic bits. On Unix, the OS-specific methods are mostly handled by GIO (if available) or GnomeVFS or QDesktopServices (if compiled with QT). Although it appears to me that the platform-specific methods are attempted first, the behavior that I have observed indicates that the platform-agnostic methods are attempted first. The behavior that I have observed is that each of the following methods are attempted, one at a time, until one of them is successful. The methods that Thunderbird attempts are (in order):

mimeTypes.rdf (Thunderbird - All Versions?)

The mimeTypes.rdf file contains information for the “Helper Applications” which are used to open external content. Existing entries can be adjusted in on the Incoming tab of the Attachments pane of the Preferences window (Edit -> Preferences -> Attachments -> Incoming), or simply on the Attachments pane in earlier versions. Unfortunately, there is no way to add new file-type associations in the preferences window.

To add an entry for a scheme that does not appear in the list (e.g. “http” or “https”), use the following process:

  1. Open the configuration editor (about:config) which can be accessed through Edit -> Preferences -> Advanced -> Config Editor....
  2. Change network.protocol-handler.warn-external.<protocol> to true for each of the protocols that you wish to configure by double-clicking on the preference. (e.g. change network.protocol-handler.warn-external.http to true to configure the program for http URLs).
  3. Click on a URL in an email in Thunderbird. Thunderbird will prompt for the application to use to open the link. Select the desired program and check the option to remember the selection.

Note: Raman Gupta made a great suggestion that choosing /usr/bin/xdg-open as the preferred application will force Thunderbird to use the XDG MIME action (the desktop manager’s default) if it wouldn’t otherwise do so.

Advanced users can also view/edit the mimeTypes.rdf file directly, although this is not recommended. The mimeTypes.rdf file is stored in the profile directory which can be found through Help -> Troubleshooting Information. It is usually located at ~/.mozilla/thunderbird/XXXXXXXX.default/mimeTypes.rdf where Xs are replaced by random characters and default may be replaced by another profile name, if named differently. The program association will be stored as an RDF Description for urn:scheme:externalApplication:<protocol> with an NC:path containing the application to run.

XDG MIME action (Thunderbird > ~4)

The default browser is chosen based on the information in the XDG MIME database, as specified in the XDG MIME Applications Associations spec. Thunderbird looks up the program associated with the URL pseudo-MIME type (e.g. x-scheme-handler/http for HTTP URLs) or the attachment MIME type (e.g. text/html).

The Applications Associations database can often be changed using the “Default/Preferred Applications” or “File Associations” GUI configuration tool for your desktop environment. The database can also be queried and modified using the xdg-mime command-line tool as follows:

# Query the current default for HTTP URLs
xdg-mime query default x-scheme-handler/http
# Set default program for HTTP URLs to Firefox
xdg-mime default firefox.desktop x-scheme-handler/http

To find the .desktop file for your desired browser, look in /usr/share/applications (for system-wide applications) or ~/.local/share/applications (for user applications).

If xdg-mime is not available, the defaults can be changed by editing mimeapps.list as described in the XDG MIME Applications Associations spec and using information from /usr/share/applications/mimeinfo.cache for reference. For example, the default program for HTTP URLs can be set as follows:

[Default Applications]

Gconf (Thunderbird > ~3 with GNOME)

When running in a GNOME environment (if the GNOME libraries are present), Thunderbird attempts to determine the default browser based on the preferences stored in Gconf. Thunderbird uses the following preferences for URLs of various types:


If the command string contains %s, it will be substituted with the URL being opened.

The information may be modified and queried using gconftool-2 on the command-line as follows:

# Query the current command for http
gconftool-2 --get /desktop/gnome/url-handlers/http/command
# Set the command for http to firefox
gconftool-2 --set /desktop/gnome/url-handlers/http/command \
    --type string 'firefox "%s"'

The information can also be queried and modified using a graphical program such as gconf-editor.

Thunderbird Preferences System (e.g. prefs.js) (Thunderbird - Old Versions?)

In all versions of Thunderbird, the default browser may be determined based on the settings in the Thunderbird preferences. Preferences can be edited by opening the configuration editor (about:config) which can be accessed through Edit -> Preferences -> Advanced -> Config Editor.... Setting a default protocol handler requires two preferences to be specified. First, network.protocol-handler.external.<protocol> must be true to indicate that the protocol should be handled by an external program. Next,<protocol> must be set to the name/path of the program which should be run to handle the URL. The following preferences may need to be set/changed:

Note that these preferences can be managed across multiple machines or made permanent by editing the user.js file. This is not recommended for normal situations but is mentioned here for completeness.

Update: After examining the Thunderbird sources, I am doubtful about whether this method is still attempted in recent versions of Thunderbird. There are no references to in the Thunderbird sources and I didn’t find any code which looks like it accesses these preferences.

Proper Documentation

The process of changing the default browser is documented on the mozillaZine Wiki. Unfortunately, the page has not been updated since July 2010 and my requests for an account have been silently ignored for weeks. If any reader has the ability to edit that page, I highly encourage you to do so.

Alternatively, this information should probably be posted on the Thunderbird Messaging KB or Mozilla Wiki. I have not yet had the time to rework this post into a suitable format for posting in either location. If someone would like to make the changes, I’d be happy to assist.

Article Changes


  • Added mention of desktop environment GUI configuration tools for MIME DB.
  • Updated mimeapps.list location from ~/.local/share/applications/mimeapps.list to $XDG_CONFIG_HOME/mimeapps.list since the previous location is described as “for compatibility, deprecated” in the spec and is less frequently used.


  • Added and clarified lots of information about mimeTypes.rdf and corrected the order in which mimeTypes.rdf is consulted based on input from Raman Gupta.
  • Added a brief discussion of how handlers are determined programmatically with references to the Thunderbird sources.
  • Added a note that the prefs.js method may not be used anymore.