Rpm-ostree quirks

I know I did not reboot after hotfix, simply because I use a post-install script, so I cannot reboot in between.

But hotfix is the only way to apply Firefox default preferences globally. Also see here and the link to Mozilla documentation: Apply custom global gnome and firefox configuration - /etc and /usr/lib64 folders in Silverblue
You need to write to /usr/lib64/firefox/ which is a read-only file system.
The file I create should be persistent using --hotfix, even after upgrades, if I understood this reply correctly: How to install Gnome Extensions globally and allow automatic updates? - #10 by jakfrost

@siosm I am no expert but the recommendation in other topics here is if you go for Firefox Flathub edition, remove override is the way to go…