I wanted to boost the size of the text and images in Firefox for my mother. Firefox has a feature called Zoom that lets you do this on individual pages. Zoom can even be tweaked a little, but I couldn't find a way to set the default zoom to anything other than 100%, until I found a useful add-on Default FullZoom Level:

"Two of the biggest complaints about Firefox’s zooming feature are the inabilities to save zoom levels for future browsing sessions and to set a different zoom level for all websites by default.

Default FullZoom Level is a handy add-on for Firefox 3.6 and later that offers to resolve both issues and more."
Source: Change The Firefox Default Zoom Level