If you use Firefox and can not find that perfect compact theme, fear not, we can easily just change the size of everything!
In address bar go to about:config. Search in there for the setting layout.css.devPixelsPerPx. For default size set it to -1.0 or 1.0. I usually set mine around 0.9 to 0.95 for slightly better use of screen real estate.
I may make a simple Firefox addon later at some point to make this feature more accessible.