Many users are unaware that it is possible to adjust the text size of websites which are designed to allow this or, maybe I should say, not specifically designed to prevent it. On some of my sites I allow users to change the size of the main body text only, not the size of the headings, as I have found that changing both together can alter the page layout so dramatically that all the context is lost.
View > Text Size > Larger / Smaller
Internet Explorer provides five choices. Most websites are designed to be viewed at Medium size, thereby offering users two larger and two smaller options.
View > Text Size > Increase / Decrease
Firefox only offers the option to increase or decrease the text size, without pre-selecting a 'medium' size. This can cause problems if users regularly visit sites which require a text size adjustment as they then have no easy way to re-select a 'medium' setting.