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.
Of course, that all depends on how much you need to zoom in. For me, that's about 6x.