I'm unaware of what exactly is going on in your situation but i only have 3 suggestions

1. use a bootable usb drive that has portable apps. use this for browsing as only your usb drive is being use and your not actually using the browser on the computer

2. To really cover your files on the computer mind you if this is your computer I highly recommend True Crypt you can have high level encryption. you can find details here http://www.truecrypt.org/

3. if even after this your still need more I suggest DBAN, this will pretty much erase your hard drive. Can they find files, maybe but if so its gonna make it harder to go through and find them

after all of this if you still require more then you must be in serious trouble to really need to go through the 3rd opinion

Does CCleaner work with MAC?

Uninstall your browser and any "offensive" files or folders.

Run a program like "File Shreddar 2.0" to clean all "empty space" on the drive using highest level of security.

Re-install your browser and set-up non-offensive bookmarks and stop worrying.

Regards

Edited October 18, 200916 yr by Martian

The security option in Firefox are fine but don't remove all traces.

You should delete all caches you will find in User/Library/Caches.

You don't need special software to wipe-out your date. You can use Apple's built-in secure version of emptying the trash. This overwrites the files several times (7?) with zeros. With the software "Disk Utility" you can securely delete the free space (in case you missed wiping files). These procedures take some time of course (up to several hours).

Apple's method is very secure. Only forensic specialist with a lot of time and very special, not commercially available software, have some chance to recover the data. Some month ago, a police officer told me that they cannot recover data which was even overwritten only 1 times.