It shouldn't do "Back" when your cursor is in a text editing field, but if it leaves the field and you press backspace, boom, you went back. I feel your pain and disable it myself. Here's how:

(1) In a new tab, type or paste about:config in the address bar and press Enter/Return. Click the button promising to be careful or accepting the risk.

(2) In the search box above the list, type or paste back and pause while the list is filtered

(3) Double-click the browser.backspace_action preference and change the value to 2 and click OK.

Two isn't a magic number, any value other than 0 or 1 causes the key to do nothing (outside of a text editing field). See: http://kb.mozillazine.org/Browser.backspace_action

Thanks.

I knew that it wasn't doing it all the time, but I could never figure out exactly when it did and didn't do it. It seemed to be doing it when I hit the backspace key several times quickly -- as in backspacing out a word before typing something else. I never thought about where the "cursor" was located because when I'm typing text I really don't care where the mouse pointer is. I have to hands on the keyboard and I usually expect the mouse pointer to stay wherever it was last - which might or might not be at the start of the text box that I am typing in.

This seems like a "feature" that ought to be easier to enable and disable, rather than requiring the Mozilla equivalent of a Windoze Registry Hack.

The "feature" is successfully disabled now -- I've tested it a couple of times while typing this reply and Firefox hasn't tried to leave the page.