A big improvement was made by some adaptative themes which take system look and icons:
GNOMErunner/GTK Revived (Freedesktop)
Also, GNOMErunner. For this there is also GNOMErunner Options.
The first integrates the system by default. If you use the GNOMErunner Options addon, be sure to set 'Freedesktop' under icons tab to get the system icons in the browser.
This theme in KDE:
In elementaryOS:
Also,the simple Firefox-like themes (that come on top of the default complete one) are now also available for Seamonkey: