Font size and zoom - increase the size of web pages
You can "zoom" all sites using this add-on: NoSquint. After setting your global zoom level, you can still fine-tune the zoom on individual sites.
After installing NoSquint, click the % on the Add-on Bar to call up site preferences, then click the Global Settings button, then the Zooming tab.
If you don't have the Add-on Bar displayed, you can right-click the new tab button ("+") or a blank area of the tab bar to the right of it and choose Add-on Bar from the shortcut menu.
You can experiment with sizes larger than 125%, but I wouldn't go much higher than 150% unless you have a large high-resolution monitor.
If there are particular sites that still present a problem, feel free to post their URLs.
You can find more information about the zoom features Firefox offers in these articles: