Yes, zoom is great in OS X. I admit I haven't tried it in Firefox, I will now.
Whenever I encounter a site that doesn't render properly with large fonts, I simply turn off the minimum font size in Firefox and use the OS X Accessibility keyboard shortcuts to zoom in. This works well, but it's annoying to have to keep switching around like that. E.g. it would be nice to have faster access to the Firefox submenu that controls minimum font size.
Whenever I encounter a site that doesn't render properly with large fonts, I simply turn off the minimum font size in Firefox and use the OS X Accessibility keyboard shortcuts to zoom in. This works well, but it's annoying to have to keep switching around like that. E.g. it would be nice to have faster access to the Firefox submenu that controls minimum font size.