The attributes of firefox might be changed when updating it. Just open a shell, and type:
chmod 777 /diroffirefox/
to let it be read, written executed and read by all users and usergroups
You can also right click the preferred files, select properties and then change the mods for it (there should be a button called 'advanced' where you can set all the options of access to each user(group). Make sure that 'users' can read and execute it.