i have the same problem in 2.19.900 -.-
the option from firefox "Saved Form Information" deled all my passwords (signons2 & signons3 file) :/
for my plasure i have a backup from my profil files but another people dont have^^
Windows XP Pro SP3 32bit
Firefox 3.0.10
CCleaner 2.19.900
EDIT: i tested on my other pc -> im runing some tests and every time ccleaner deled it...