Disbling JavaScript in Thunderbird:
Edit --> Preferences --> Advanced --> General,
search for 'java', set 'javascript.enabled' to 'false'.
Settings -> Scrolling -> Always show scrollbars
And:
Settings -> Config editor -> search: widget.gtk.overlay-scrollbars.enabled -> False
More;
Thunderbird
about:config Modifications
Not all settings are available in 'about:config' or 'config editor'. For
some things, you need to edit 'prefs.js';
Modify
Thunderbird settings
A list unsubscribe plugin.