Actually, you cannot stop them from deleting your history stuff, alternatively, you can change the
profile you use so that the history & cache data is saved specifically for your profile only.
To change profile:
1. Got to
Start->Run->firefox.exe -p
This -p is for the
Profile Manager
( Do this only when you're not running firefox)
2. In the dialog box that appears, click on
New..
3. Enter the name of the profile,etc. and save it
( Do not make it the default )
Now, whenever anybody else uses firefox, they will be using the default profile whereas your data will be kept safe in the other created profile
Note: To ensure that your stuff remains safe, you will have to select the profile everytime before running firefox. (To gain something, you've got to work for it)
Any more problems, have a look at this thread:
Profiles in Firefox