Pixlr Editor doesn't come installed with preset fonts. Instead, it grabs the fonts that are installed on your computer and lets you utilize those fonts through the Pixlr interface. Most fonts will work, although some may not. Fonts are provided by many different companies — some fonts are free and some require payment. While Pixlr isn't able to work with every font out there, in most cases you can simply install and use your own custom fonts.
The details about how to install fonts varies from computer to computer, but the font provider Linotype has some basic tutorials that can help you with Windows or Mac font installation.
Note: You'll need to download and install fonts and then re-launch your browser before Pixlr will be able to utilize new fonts.