There are a lot of hidden preferences in Firefox that are available by entering about:config in the browser’s address bar and pressing the Enter or Return key. Here are some tweaks that I find particularly helpful.
To change a preference in the list, double-click on it. To filter the list, type any part of the preference name in the Filter box.
By default on Windows and Mac OS X, hitting the backspace key causes Firefox to go back one page in the browser’s history. Changing this setting’s value to anything greater than 1 (Firefox on Linux defaults to a value of 2) will disable this behavior to prevent accidentally changing pages while typing in a form or Flash program.
The default value of 1 enables spell checking in text areas. To have Firefox show incorrect spelling in text input fields of forms as well, change this value to 2.
Changes to these settings take effect immediately. The spell check one may require that you reload a page that was open before the setting was changed for it to take effect.