"Starting with Firefox 22 the web browser is now respecting the display scaling of the operating system which may have changed how the browser UI and websites are displayed on user systems. The linked article highlights how you can change the size of fonts in the browser so that they appear larger or smaller on the screen.
"But what if you have issues with the font size on some websites only? Say sites that use a font that is too small to read. While you can use the browser's zooming feature to zoom in on the text by holding down the Ctrl-key and using the mouse wheel to zoom in or out, it is not really that comfortable of an option if you experience the issue on many sites. ..."
Source: Ghacks Technology Newsletter
Details No Small Text for Firefox: set a minimum font size for all web contents