How do I set the default browser for xdg-open on Centos 7 if xdg-settings has no desktop environment

There are many questions similar to mine (e.g. xdg-open not open default browser or xdgutils - xdg-settings not setting default-web-browser in gentoo, but none of the answers helped in my case. Therefor I ask for my particular situation:

On Centos 7 I have no free desktop manager running, I just run some X11 applications (like VS Code) from the command line where the DISPLAY variable is set to the X server on the (Windows) machine I connect from.

On the Centos machine I have two browsers installed, firefox and google-chrome. I can start both browsers just by typing firefox resp. google-chrome in the bash terminal.

xdg-open is available and it opens links in google-chrome - as does VS Code. However I want to change this to firefox.

I tried:

Ticking "Default browser" in Firefox's GUI preferences.
Using xdg-settings, but
```
xdg-settings get default-web-browser 
```
returns "xdg-settings: unknown desktop environment"
Setting $BROWSER. In bash I issued
```
export BROWSER=firefox
```
but still google-chrome is started by xdg-open

How can I set in this environment the default browser to firefox?

Note: Strangely on another machine with Centos 6 (and "no desktop environment" either) the export BROWSER method works!

Solution

The desired behavior can be set in the mimeapps.list configuration files described in the XDG MIME Applications specification.

TLDR: In order to configure firefox as the default browser for your user create ~/.config/mimeapps.list containing the following lines:

[Default Applications]
x-scheme-handler/http=firefox.desktop
x-scheme-handler/https=firefox.desktop
x-scheme-handler/ftp=firefox.desktop
x-scheme-handler/chrome=firefox.desktop
text/html=firefox.desktop
application/x-extension-htm=firefox.desktop
application/x-extension-html=firefox.desktop
application/x-extension-shtml=firefox.desktop
application/xhtml+xml=firefox.desktop
application/x-extension-xhtml=firefox.desktop
application/x-extension-xht=firefox.desktop

Details: xdg-utils like xdg-open(1) and xdg-mime(1) look for this file in the locations listed under the File name and location section of this specification:

$XDG_CONFIG_HOME/$desktop-mimeapps.list user overrides, desktop-specific (for advanced users)
$XDG_CONFIG_HOME/mimeapps.list user overrides (recommended location for user configuration GUIs)
$XDG_CONFIG_DIRS/$desktop-mimeapps.list sysadmin and ISV overrides, desktop-specific
$XDG_CONFIG_DIRS/mimeapps.list sysadmin and ISV overrides
$XDG_DATA_HOME/applications/$desktop-mimeapps.list for completeness, deprecated, desktop-specific
$XDG_DATA_HOME/applications/mimeapps.list for compatibility, deprecated
$XDG_DATA_DIRS/applications/$desktop-mimeapps.list distribution-provided defaults, desktop-specific
$XDG_DATA_DIRS/applications/mimeapps.list distribution-provided defaults

The locations for the $XDG variables are governed by the XDG Base Directory specification. If you want to figure out where xdg-utils are looking for configuration in your particular case, run them with the XDG_UTILS_DEBUG_LEVEL environment variable like so:

$ XDG_UTILS_DEBUG_LEVEL=10 xdg-open 'https://www.example.com'
...
Checking /home/USERNAME/.config/mimeapps.list
...