Libtiff is not required by Firefox (Firefox). It is required by oy one of the GTK+ packages. GTK+ would actually work just fine without TIFF support. But I figure that the maintainers of the GTK+ port in FreeBSD figured it would be a nice idea for GTK+ programs to have TIFF support.
So you could potentially have two copies of GTK+ on the system. But that would in fact involve a longer build time. Assuming you do want TIFF support in some other GTK+ programs, why not have it just enabled in GTK+?
So either fix it the "Ubuntu" way (build GTK+ with TIFF support for everybody) or the "Gentoo" way (build GTK+ without TIFF support for everybody. Keeping multiple copies increases your overall build time (not to mention other types of overheads).
Oh, and do you mean that Firefox (and Chromium, likewise) have this nice habit of not reusing system components? Well, this is a know issue. In the works, I guess.