Go to View > Text size and select your desired text size setting (eg, larger, smaller).
Go to View > Text size and increase/decrease using Ctrl and + or -.
If you have a scroll wheel on your mouse, you can hold down Ctrl and scroll back or forth to increase or decrease the font size in both IE and Firefox.
Click on the spanner symbol in the top right hand side of the browser. Next to the word 'zoom' select the + or - symbols to enlarge or shrink the text.