There used to be a very useful down arrow button next to the back button which brought up recent history, but it was removed in Firefox version 4. For the life of me, I don't understand why. If you have an older version, just click the down arrow button and select the page you were at before you loaded the offending page.

If you have Firefox v 4 or 5, the best workaround I know of is to do an <alt-s> from the keyboard, which will bring down a history list from which you can select the page you were at before you got trapped. Alternatively, you can bring up the old style menu bar by pressing either Alt or F10, and then drop down the history menu by clicking on it with your mouse. If you want to make that menu bar permanent, you can select View -> Toolbars -> Menu Bar.