Is it really Firefox?
If you open Firefox and open Edit, Preferences, Content you can see what font Firefox is using. The next time this happens in Firefox do the same and see if you are in fact changing Firefox.
You have not told us which linux on which laptop you are using. You might do that, and if you have not, google for changing fonts, fonts, etc. for your linux distro. Read this for your next post.
http://www.linuxquestions.org/linux/...Ask_a_Question
Good Luck