If some hard drive or Windows problem meant you had to download and reinstall Firefox then that wouldn't take long, maybe just two or three minutes - but what about the rest of your profile data? Your bookmarks, saved login data, form details? The program settings? All the extensions you've installed? If you like to customise the browser then it could take a very long time to restore your previous setup.
Unless, that is, you've been a regular user of MozBackup. This tiny program makes it easy to back up all your Firefox profile details (settings, bookmarks, history, user styles, extensions, passwords, cookies, form details, downloaded file list and certificates). It can also back up profiles for other Mozilla and related apps: Thunderbird, Sunbird, Flock, Postbox, SeaMonkey, Mozilla Suite, Spicebird, Songbird, Netscape and Wyzo. And if you do ever have a hard drive disaster, and your valuable data is lost, then the program can restore your latest backup in seconds.