MaxKoll

Fixed a mistake (`shell_process.exitValue` -> `exitValue`) and some formatting details.

Some restructuring and reformatting to improve clarity. Program flow did not change.

Could this change in FF 135 have caused the extension to break? [User Interaction Tracking should be considered for initial history entries](https://bugzilla.mozilla.org/show_bug.cgi?id=1924861) This feature, requested three years ago, would probably...

I just found out that `Navigation.canGoBack` was implemented three days ago and is scheduled to be available in FF 137 (to be released on April 1): [Implement 'Navigation.canGoBack' and 'Navigation.canGoForward'](https://bugzilla.mozilla.org/show_bug.cgi?id=1896629)...

> This will not allow to activate the back button, only let us track the current history entry. True, and I have no idea how to accomplish this. For me...