Taming Thunderbird

Disable JavaScript

Disbling JavaScript in Thunderbird:
Edit --> Preferences --> Advanced --> General,
search for 'java', set 'javascript.enabled' to 'false'.

Show scrollbars

Settings -> Scrolling -> Always show scrollbars
And:
Settings -> Config editor -> search: widget.gtk.overlay-scrollbars.enabled -> False

About config / Config editor

More;
Thunderbird about:config Modifications

Edit prefs.js

Not all settings are available in 'about:config' or 'config editor'. For some things, you need to edit 'prefs.js';
Modify Thunderbird settings