Step 1.) Upgrade to Debian 11 first
The process to go to Debian 12 is not as smooth as 11, when trying to upgrade from Debian 10. In fact, it doesn't work directly, so you'll first need to follow this guide to update to Debian 11, reboot and come back here if successful.
Step 2.) Update sources.list
Update your /etc/apt/sources.list like this:
deb http://........
(firefox:9562): LIBDBUSMENU-GLIB-WARNING **: Unable to get session bus: Failed to execute child process "dbus-launch" (No such file or directory)
ExceptionHandler::GenerateDump cloned child 9743
ExceptionHandler::WaitForContinueSignal waiting for continue signal...
ExceptionHandler::SendContinueSignalToChild sent continue signal to child
[Parent 9562, Gecko_IOThread] WARNING: pipe error (40): Connection reset by peer: file /build/firefox-EymEXX/fire........