Rated 5 out of 5 stars

ChmZoomer gives you handy full page zoom functionality in all other browser-based applications, such as Chm files, e-Books, RSS Reader and so on. That is just like in IE7/8.
The shortcut keys are the same as IE and Firefox. Press [CTRL +] or [CTRL Mouse wheel up] to zoom in. Press [CTRL -] or [CTRL Mouse wheel down] to zoom out. Reset zoom to 100% with Ctrl-0.
See http://www.goldgingko.com/chmzoomer/