I don't know if this is related to CCleaner; I don't think so. I have CCleaner and Firefox installed on many machines (~10) and have only had this problem on one. It turned out the update URL was corrupt. I fixed it as follows.
Start Firefox, open about:config and find app.update.url. Set the value to the following.
https://aus2.mozilla.org/update/1/%PRODUCT%...NEL%/update.xml
Now do Help->Check for Updates...
Hope this helps.
Best regards,
StacyJ