I have version 99.0.1-1 (stable branch) on Kubuntu 22.04 (personal laptop & gaming desktop).
Even doing manually doesn’t work.
I set the about :config option widget.use-xdg-desktop-portal.file-picker to default (“2”). no dice.
There’s a bug report about this, but I’m not able to reproduce it in a Kubuntu 22.04 VM.
Can you elaborate on how you’re trying to set firefox as your default browser? Using the system settings, or in firefox’s preferences?
I just tested the latter (setting the default browser from within firefox, in about:preferences), and indeed it’s not working.
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Firefox calls xdg-settings set default-web-browser firefox_firefox.desktop, and I verified that this call works, the default XDG app is changed. However for some reason firefox isn’t able to see that the change happened, and neither is the KDE settings app.
In that Kubuntu 22.04 VM:
$ kreadconfig5 --file kdeglobals --group General --key BrowserApplication
firefox_firefox.desktop
Yet the settings application still displays Kate as my default web browser. That looks like a problem with the settings application itself?
Not in my case. Mine was already set to Firefox (can’t recall if it’s something I forced).
Firefox still seems to think it’s not the default
To check whether it’s set as the default browser, firefox issues this call:
xdg-settings check default-web-browser firefox.desktop
Which in stock Ubuntu replies “yes”, as expected. But in Kubuntu I’m seeing this:
Error org.freedesktop.DBus.Error.Failed: cannot check "default-web-browser" setting: /usr/bin/xdg-settings: 734: Bad substitution
The offending line is:
if [ x"!" == x"${browser:0:1}" ]; then
where the browser variable contains firefox_firefox.desktop, in our case.
Parameter expansion for string extraction is a bash-specific thing, but the default interpreter for /usr/bin/xdg-settings is /bin/sh (an alias to /usr/bin/dash), which indeed doesn’t do this.
And sure enough, if I change the default interpreter to /bin/bash at the top of /usr/bin/xdg-settings, the firefox snap now detects that it is the default browser.
That’s a Kubuntu-specific problem with the xdg-utils package, for which I’ve filed bug #1970594.
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Thanks for your help 
It says it’s fixed, but I’m still experiencing the issue
$ kreadconfig5 --file kdeglobals --group General --key BrowserApplication
firefox.desktop
$ xdg-settings check default-web-browser firefox.desktop
/usr/bin/xdg-settings: 734: [: x!: unexpected operator
no
Have you used firefox to re-set it as default after you received the patched xdg-utils? if you haven’t then you need to do so for xdg-settings to correctly report that firefox is default.
Still doesn’t work if I use firefox_firefox.desktop. I still get asked.
$ kreadconfig5 --file kdeglobals --group General --key BrowserApplication
firefox_firefox.desktop
$ xdg-settings check default-web-browser firefox_firefox.desktop
/usr/bin/xdg-settings: 734: [: x!: unexpected operator
yes
Patch your /usr/bin/xdg-settings as follows:
--- xdg-settings.orig 2022-07-29 16:47:56.186059909 +0200
+++ xdg-settings 2022-07-29 16:47:32.669780678 +0200
@@ -731,7 +731,7 @@
binary="`resolve_kde_browser`"
# The browser may contain a relative entry to the binary starting with !
- if [ x"!" == x"$(printf %.1s "$browser")" ]; then
+ if [ x"!" = x"$(printf %.1s "$browser")" ]; then
# get the full path
browser="`binary_to_desktop_file ${browser:1}`"
binary="`desktop_file_to_binary $browser`"
@@ -1087,7 +1087,7 @@
if [ x"$1" = "mailto" ]; then
binary="`read_kde_config emaildefaults PROFILE_Default EmailClient`"
# The field may contain a relative entry to the binary starting with !
- if [ x"!" == x"$(printf %.1s "$binary")" ]; then
+ if [ x"!" = x"$(printf %.1s "$binary")" ]; then
# get the full path
desktop_file="`binary_to_desktop_file ${binary:1}`"
binary="`desktop_file_to_binary $desktop_file`"
See my comment at https://gitlab.freedesktop.org/xdg/xdg-utils/-/merge_requests/52#note_1488548
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