It's tricky to seamlessly override the fonts websites choose.

• Firefox makes it easy to simply substitute a handful of selected fonts for all website fonts, but this will not always work well with the layout.
• The zoom feature will enlarge all characters making them easier to read, but sites that do not flex to fit the window will get a horizontal scroll bar. (Font size and zoom - increase the size of web pages)
• You can create custom style rules for the websites that most need improved legibility, but this will take some time to set up, and a bit of experimentation.

Perhaps there is an add-on that improves readability without having to make a lot of compromises? I haven't tried any of them myself.

Note that the NoSquint extension can set a default font.

• NoSquint: https://addons.mozilla.org/firefox/addon/nosquint/