Try reloading the page (press F5), closing and re-opening the tab (Ctrl+W, then Ctrl+Shift+T) or restarting firefox.
The google-page (I'm assuming you mean the mostly blank frontpage google.com) is a very simple-coded page, and in terms of html, there should be no reason for it to change anything about the cursor. Please make sure the site you are on is indeed google.com and not a clever redirection to a similar site with malicious intents (as in google.con. No, that's not a real site).
Common reasons for the cursor to disappear are usually either scripts/flash in the browser, applications that do not respond, or applications which (intentionally or accidentally) hide the cursor (for instance, a game running in fullscreen). None of these are on the google frontpage.
If you are viewing the google page in a firefox tab, but other tabs in firefox show the cursor, I'm out of answers. But if you are viewing it in another browser(-instance), then maybe a restart or reinstall might help.