I have noted for some time now that ccleaner has some sort of problem when it comes to cleaning Firefox cache files. This goes back literally years and many many versions of ccleaner and Firefox. After searching and reviewing forum files it is obvious that this is a recurring and known issue and yet apparently has never been addressed by the authors. I can find post after post after post from people experiencing the same issue and the responses are universally insipid, blaming it on everything from not cleaning often enough to antivirus to who knows - stage of the moon?
There clearly is and has been a problem for some time. Why won't Piniform address it?
piriform can only address that which they can recreate. Granting there are a small number of those with slow cache cleans, the majority of people do not experience this issue.
That said I can think of a few reasons for this
Large Cache
Unclosed Mozilla product (including background processes which did not properly close)
Corrupted cache database file
Cache Compaction (via ccleaner) is turn on
older version of firefox
any or none of the above may contribute to the rare "slow cache cleans"
I personally always suggest that, if it is capable, users let browsers clean themselves (I use an add-on in Firefox) and to use CCleaner as a supplemental cleaning tool.
That's the only think that makes CCleaner slow on my system cleaning Firefox ("Firefox clones such as Pale Moon"), CCleaner is not particularly quick at all when 'Firefox -> Compact Databases' is ticked/enabled and when it deems it necessary to compact them.