It would be cool if this could be tested on other OS with different models of mice. According to a quick Google search, buttons 6/7 and 8/9 can also be used for back/forward, and GDK really only supports 3 buttons (or 5 according to docs).
Another option, though perhaps overkill, would be to add a small GUI to the preferences to allow binding mouse buttons, somewhat similar to the keybindings. Or at least a "Various" setting so the can change manually.