+ increases the browser font size, and - decreases it. Works in both Internet Explorer and Firefox.
Your browser's setting may also be affecting the font size you see. Check your default settings.
Finally, it's getting more and more difficult to design web pages that work for everyone. There are still a few 800px wide screens out there, most folks are using 1024px screens, but newer computers are 1400px and greater (mine is 1680px wide, and they go higher). Some web sites read your screen size and serve up a corresponding style sheet, some don't.
So when you hit a site that is tough to read, just resize the text yourself - I have to do it all the time, and my vision is still good enough that I can pretend I don't need glasses.
JAC