Revise Your History
Firefox offers a way to delete selected pages from both your browsing history and your address bar history while retaining everything else.
To delete an entry in the address bar's dropdown menu of recently-visited pages, point to the item with your mouse, but instead of clicking it, press Delete.
To edit your browsing history, press Ctrl-H to open the History sidebar. Right-click the page you want to remove and select Delete. If you want to delete a bunch of pages, all adjacent to each other (as they will be if they're from the same site), right-click the first one and select Delete. When it disappears, the one below will be highlighted, continue to press Delete on the keyboard until they're all gone.
Safely Store Passwords
It's good security practice to use a different password for each site you visit, though it can be difficult to remember them all. Unfortunately, Firefox's password manager is not really safe unless you use its Master Password option.
To set this up, select Tools, Options and click the Security icon. Check Remember passwords for sites and Use a master password. In the Change Master Password dialog box, enter a password that you can remember but that no one else can guess. Check out this Windows Tips column for advice on selecting a safe, but memorable, password.
Once you've set your master password, managing your Web passwords is easy: Whenever you enter a password on a page, a dialog box will ask if you want Firefox to remember it. The next time you return to that page, the browser will automatically fill in the password--with one important exception. If this is the first time you've gone to such a page since you last launched Firefox, you'll have to enter your master password first.