Either the Default FullZoom Level or NoSquint extensions may be of use to you. They both allow you to set default zoom levels for all web sites.

Another possibility is changing a pref to increase the size of the user interface and on web pages.

You can increase the String pref layout.css.devPixelsPerPx from 1.0 to 2.0 in 0.1 steps to see what works best.

In Firefox 3.6 and later this pref is a String value parsed to a float and allows to fine tune the dimensions of all elements more precisely.

To open the about:config page, type about:config in the location (address) bar and press the "Enter" key, just like you type the url of a website to open a website.
If you see a warning then you can confirm that you want to access that page.

Did you try holding Control key and click + or Control key and click - to return to former size? This works on XP.