For some time, I have had problems with adsense (not just my own sites, but MANY sites) not showing up.
It's only been a problem in Firefox.
I've searched the net, and it seems that many people are having this problem, but there are no solutions.
I'm posting this, in case it helps someone ;)
Adsense-ads sometimes not showing up in firefox, but always works in other browsers
MY Solution, which worked!:
Disable FireFTP version: 1.99.5 as Addon.