If you have a hard time reading some websites then you should download the NoSquint Firefox extension. Once installed you can adjust the default text zoom level so that the text will appear larger or smaller by default. You can also have it change the size of the text based on the domain you are visiting.
This is one extension that comes in handy for certain sites. However, I don’t think I actually use it for the purpose it was designed for. When I visit sites, like Google, I have it shrink the text down to 95% because it is still readable and more fits on my screen at the same time then. I have a lot of custom domains setup to automatically adjust the size of the text each time I visit them.
Whether you use the extension to make text larger or smaller I’m sure you’ll find it useful on at least a few sites.