For your information, free software tools that have unwanted extras bundled with it usually hijack your browser. So install free software with caution. Here is how to lock your Firefox’s homepage to prevent hijacking.
It is not easy to lock your Firefox’s homepage, however, there is a trick that you can try to lock the homepage. To do this, you have administrative rights to your Windows PC.
1. Open Notepad and type: pref (“browser.startup.homepage”, http://www.myhomepage.com;lockpref (“browser.startup.homepage”, http://www.myhomepage.com);
Note: Replace myhomepage.com with the homepage URL of your choice, for instance http://www.google.com.
2. Name the file ‘mozilla.cfg’ and save it. Now copy and paste this file to your Firefox folder at C:\ program Files\Mozilla Firefox.
3. Now create another Notepad file and enter the following text: pref(“general.config.filename”, “Mozilla.cfg”);
(Make sure you include the semi-colon).
Name the file ‘local-settings.js’ and save it. Now, copy and paste this file at Mozilla Firefox – default – pref folder.
Finally, restart Firefox and your home page should now be locked against changes.
How to unlock your homepage: You can unlock it by deleting the contents of the mozilla.cfg file. (source)