I tried enabling native KDE dialog integration with Firefox using xdg-desktop-portal-kde, and noticed that Firefox keeps asking about being set as default browser on each startup.

I installed:
xdg-desktop-portal 1.0.3
xdg-desktop-portal-kde 5.14.3

Set in $HOME/.profile:

export GTK_USE_PORTAL=1

Applications > Default Applications > Web Browser already set to Firefox.

When GTK_USE_PORTAL is not set, it's not asking after first time.

I also noticed, that Firefox normally remembers this setting after creating a NoDisplay .desktop file in $HOME/.local/share/applications

Such as:

$HOME/.local/share/applications/userapp-Firefox-IIJAUZ.desktop

[Desktop Entry]
Encoding=UTF-8
Version=1.0
Type=Application
NoDisplay=true
Exec=/opt/mozilla/firefox/firefox-bin %u
Name=Firefox
Comment=Custom definition for Firefox

This is not visible in menu and not available to System Settings choice (because of NoDisplay=true), so I usually create another .desktop file there, to expose Firefox. Something like:

$HOME/.local/share/applications/Firefox.desktop 

[Desktop Entry]
Comment=
Exec=/opt/mozilla/firefox/firefox %u
GenericName=Mozilla Firefox
Icon=/home/user/pictures/icons/firefox.png
MimeType=application/x-www-browser;
Name=Firefox
NoDisplay=false
StartupNotify=false
Terminal=false
TerminalOptions=
Type=Application
X-KDE-SubstituteUID=false
X-KDE-Username=
Categories=Network;WebBrowser;

This way it can be selected in System Settings > Applications > Default Applications > Web Browser.

Something between these two and xdg desktop portal goes wrong I suppose.