According to Wikipedia[1], "firefox" is an alternate name for either a red panda or red fox. Also according to Wikipedia[2], a (now-removed) FAQ[3] from Mozilla itself says that the browser is named after the red panda. Other pages on Mozilla's website seem to support the idea that the name comes from the red panda[4].
1. https://en.wikipedia.org/wiki/Firefox_(disambiguation)
2. https://en.wikipedia.org/wiki/Firefox#Trademark_and_logo
3. https://www.mozilla.org/en-US/projects/firefox/firefox-name-...
4. https://www.google.com/search?newwindow=1&q=red+panda+site%3...