This is a lot later than the general discussion, but in case someone stumbles upon this while doing a search, here is what I do on my Mac with Firefox to deal with
the accidental go to previous page and loose your work when you only meant to go to the beginning of the line and go crazy because you, yet again, lost 15+ minutes typing.
Moodle 1.9: put TinyMCE into popup mode. Because there is no previous web page to go back to, you can't accidentally lose work.
Moodle 2.0+: No popup mode, and the "popup like mode" is still succeptible to the back a page problem. I installed a Firefox plugin, Customizable Shortcuts, that allows me to override the default behavior for certain key combinations. Specifically, I changed it so that the Alt+Left (using my MS natural kbd) does nothing. Problem solved.