You can use NoSquint to upscale entire pages globally (much like Firefox's built-in per-site zoom). It doesn't mess up layout like font-only scaling, though it can do that too, if you want.
The problem with scaling the entire layout is that you end up with a lot of horizontal scrolling, in a lot of cases, which is more annoying than a semi-broken layout.
Of course, that all depends on how much you need to zoom in. For me, that's about 6x.
