Presently with Firefox, the only ‘zoom’ (CTRL + +) increases the text-size only. The original Mozilla Suite (now The SeaMonkey Project) and Opera have a page zoom, that is where the entire page (text, objects and pictures) are zoomed. This was filed as bug 4821 back on April 8, 1999 and on July 25th, 2007 it was finally marked as RESOLVED FIXED.
Now before you get all excited, it is not quite done yet. The backend functionality is done which is why the bug status has been changed to RESOLVED FIXED. However, there is currently not a way to call that backend code. The user interface (UI) still needs to be developed and there is likely to be bugs resulting from that as well. Bottom line is, by the time Firefox 3 is released around the end of this year that functionality should be implemented. Highly doubtful we will see it with the M7 release at the end of this month. Chances are it will be in the M8 release scheduled for mid-September.
News Source: Mozilla Links