Since Firefox 3.0, I find the "Full Page Zoom" function very useful in daily life. The only problem with this function in Firefox is that it has no support for "default" zoom level. That is, when you navigate to the new web pages, you need to adjust the zoom level every time.
Fortunately, someone has built the extension to solve this problem. The No Squint add-on allows you to set default zoom level. And I'm living happily with it for a week so far :)