CCleaner version 5.27.5976 64 bit version crashes when trying to clean Firefox v51.0.1 "Compact Databases"
CCleaner started crashing after an update of Firefox. I tried various settings and finally narrowed it down to Firefox. In Applications / FIrefox section, if I check the Compact Databases, then CCleaner crashes when trying to clean.
If I uncheck that box, CCleaner functions OK.
It does not affect the Registry cleaning.
See attached screen shots
A few other members were having the same issue (also with compact databases). The current work around (as you've stated) is to uncheck compact databases.
The developers have been made aware of this issue.
FWIW, I have had this issue and identified that it also affects thye compact database option for Thunderbird - the Mozilla Email program too. I had to disable the compact database option for both programs to get CCleaner working again.
Confirmed here. Glad I saw these posts - I thought it was just me!
Bug still there in v5.28.6005.