To restore the default home page, Mozilla support recommends the following procedure:
- Go to the menu Edit > Preferences.
- Select the General tab.
- Click on "Restore to Default".
- Click on "Close".
Ubuntu comes with Firefox installed together with the Ubufox extension, which is the one responsible for changing the default start page from Firefox's to Ubuntu Start Page. If that's the version of Firefox you have, the above procedure will set the same Ubuntu Start Page you don't want (in other words, this procedure is valid for a vanilla install of Firefox).
To set manually the standard Firefox Start Page, just type
about:home in the field "Home Page", as suggested in other answers.
Image credit: Mozilla (see link above)