It does sound very much as if it could a problem with the backspace key sticking, as rinkuhero suggested. Try pressing that key a few times, that might free it up. Or, if that doesn't work, you can fairly easily disable the action of the backspace key in Firefox.
Open Firefox, then open a new tab -- so there's nothing to go back to in that new tab -- and in the address bar up top enter
about:config (where there's usually stuff like
http://www.kiva.org)
You'll see a warning that messing around with the configuration settings can be dangerous.
Then, having clicked OK to acknowledge the warning, enter the word
backspace into the filter field. This should bring up an entry called
browser.backspace_action. Right click on that, select "modify", change the value from 0 to 2, and press OK. Now close the tab you opened, and see if you can browse as normal in another tab.
If this does work and you can browse OK, it would confirm that there's a mechanical problem with the backspace key.