Firefox stores details of the fonts which you'd like it to use but, by default, allows websites to take control and substitute their own custom fonts. It seems as if Firefox is sometimes picking up the information from websites (and thus using their chosen fonts) but at other times using the fonts which you can pick yourself. (If you want to see what those are, click the three lines at the top right, then Options and Content. That will show you the main default font; click 'Advanced' for the full set).
The intermittent missing font information could come about because of either
(a) a problem with Firefox ; or
(b) the data being held back by your anti-virus program. (Once again Kaspersky is under suspicion here).
If you want to rule out a Firefox problem, reset it as per my previous instructions but remembering to note down the names of any extensions you're using FIRST. (Resetting Firefox to its default state removes any extensions you're using, so you'll have to reinstall them afterwards. However things like your bookmarks will remain untouched).
I still think that it's Kaspersky that's causing the problems though!