I actually hate to report this one, as I kind of prefer fixed-width tabs. But it
does say in the HIG:

"Use tabs that are proportional to the width of their labels. Don't just set all
the tabs to the same width, as this makes them harder to scan visually, and
limits the number of tabs you can fit into the notebook without scrolling."

http://developer.gnome.org/projects/gup/hig/1.0/controls.html#controls-notebooks

However, I'm entirely ignorant of how GtkNoteboooks work. Is this something
Epiphany can change, or a fixed property of the control? If the latter, I'll
raise it against gtk (and look embarrassed).