The upcoming Firefox 3 browser will include full-page zoom.
The page zoom feature request was originally filed in Bugzilla in 1999, but implementation was delayed because it wasn't considered practical in Gecko 1.8, the current version of Firefox's HTML rendering engine. Scheduled for inclusion in Firefox 3, the new version of the rendering engine—Gecko 1.9—leverages the open source Cairo vector graphics rendering framework and includes a number of extremely significant improvements to layout and rendering. The page zoom feature was implemented as part of the Gecko overhaul and will now be included in Firefox 3.
Although the page zoom feature is fully implemented, the user interface doesn't expose the page zoom functionality to the end user yet. Until menu items, toolbar buttons, or keyboard shortcuts are added to facilitate access to the page zoom feature, most users probably won't be able to make much use of the new feature.