Most web sites break down when fonts are upscaled. Sad but true. I configured my mother's firefox to do that, it works, but most of the web doesn't look good.
Which is sad. My eyes aren't very good too and they prefer bigger fonts. In fact, most people have vision problems those days.
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.
Then you learn most mobile sites disable user scrolling, because of a bug on iOS that causes non-standard zoom levels to cause text to wrap off the screen when changing orientation. So if you did this and your site isn't readable on someones device, they can't even fix it.
Which is sad. My eyes aren't very good too and they prefer bigger fonts. In fact, most people have vision problems those days.