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To add your own key binding for, let's say, ALT+Z, create this file:
<?xml version="1.0"?>
<!-- In order to work correctly, this file must be named -->
<!-- res/builtin/userHTMLBindings.xml in the mozilla directory. -->
<bindings id="htmlBindings"
xmlns="http://www.mozilla.org/xbl"
xmlns:xul="http://www.mozilla.org/keymaster/gatekeeper/there.is.only.xul">
<binding id="browserUser">
<handlers>
<handler event="keypress" key="z" modifiers="alt"
command="cmd_browserBack"/>
</handlers>
</binding>
</bindings>
A bit complicated, but it should be obvious how to change the key. (I didn't try it without a modifier, that would likely interfere with typeahead search... you'd have to experiment.)
Any mouse gesture addons available for IE?
Not that I know of. The mouse gestures for Moz clients can be found here [optimoz.mozdev.org].