In FireFox, you can either set the browser to use the font and color specified by the webmaster or use the font and color specicied by you. To set this, in the menu bar, click on Tools > Options, then the CONTENT tab (see screenshot on the left) in the "pop-up". In the second section is "Fonts and colors" where you can chose your preferred font by clicking on the "down" arrow and selecting the font from the list that had been installed in your computer. Click on the font you prefer. You can also specify the font size by clicking on the "down" arrow for the font-size and selecting your preferred font size. If you want to specify the color, click on the color button and select your text and background color. These will be the choices if the webmaster of the web pages didn't specify one.
Irrespective of what you set, you can chose the font and color the pages (what is specified by the webmaster) specify, in which case your computer will search for the first defined font, and if that is not available, the next, then the next, and so on.
A wise webmaster will specify a browser safe font family for their web pages, a list of which you can get from here:
If you are the type of surfers who go to websites where the webmasters uses unusual fonts, you may want to download and install extra list fonts into your computer. A link to one such site is given in this post Special fonts for your blog clickable graphic header without having to remove the text blog title. The link is entitled "Urban fonts". If you want more, you can always google for them using the search box above by typing in "download font" or some other search terms and ticking the (.) Web radio button.