Help reinstalling Windows 7 is a bit much to support over a public forum, and definitely outside the scope of this forum. It’s a complicated process if you’ve never done it before, and a lot can go wrong if you don’t backup/restore your data properly.
Concerning reset of Firefox, yes, you will lose data. If that data is passwords, that’s pretty serious. That is why most people manage passwords with Firefox Sync; you can reset Firefox and still retain passwords. You can use Firefox on many different devices and still have access to your passwords. I do not know how to advise you with Avast products as they are outside the scope of my experience. I recommend asking Avast how to backup and restore your passwords.
Concerning Ghostery, Adblock Plus, and related addons: unless you’ve added custom rules, such addons generally do not require backup/restore. In other words, generally, once you reinstall them you will have the same setup as before. I say generally because I do not know if you’ve customized them in such ways that Firefox Sync won’t restore settings (if you’re even using Firefox Sync). If you haven’t done any customizations to them, there’s nothing to fear… you’ll only be unprotected for the time between Firefox Reset and re-installation of those addons from addons.mozilla.org.
If you have any hesitation in doing this, I recommend seeking in-person advice of local professional at a computer repair shop.