Whenever other apps need to launch a browser, they likely invoke x-www-browser or gnome-www-browser. If other apps aren't invoking Firefox, you may need to manually set its priority.
Open a Terminal (Ctrl-Alt-T), then copy-and-paste the following command-line to check the x-www-browser setting:
sudo update-alternatives --config x-www-browser
If Firefox isn't on the list of installed browsers, add it with the following Terminal command:
sudo update-alternatives --install /usr/bin/x-www-browser x-www-browser /usr/bin/firefox 250
Repeat the first command above to confirm Firefox's position on the x-www-browser list.
Likewise, check the gnome-www-browser setting:
sudo update-alternatives --config gnome-www-browser
If needed, add Firefox to the gnome-www-browser list with this command-line:
sudo update-alternatives --install /usr/bin/gnome-www-browser gnome-www-browser /usr/bin/firefox 250
As default browser, the following Terminal command pops open Firefox and goes to DuckDuckGo...
xdg-open https://duckduckgo.com
Now apps such as Google Earth will invoke Firefox whenever a browser is needed.