Walter, good to see you using Shell Commands, but please remember to back up $HOME/.bcast5/Cinelerra_rc because that is where there usage is saved.

$CIN_BROWSER is by default firefox, but you can chnage this during compilation ..
or may be at runtime via env. variables, too?

As Andrew has already stated, you can set your CIN_BROWSER value before bringing up CinelerraGG as noted in the manual. You can put that in your .profile file. This is the easiest way.

16.7 Environment Variables for Customization
Environment variables are global variables in the shell which all applications can
read. They are set with a command like set VARIABLE=value or export VARIABLE=value.
Environment variables can be viewed with a command like env. The values set
can be removed with unset VARIABLE.
...
CIN_BROWSER name of browser to use by Shell Cmds options