While the introduction of multi-process capabilities to Firefox should be seamless and invisible to Web content, there are some exceptions and subtle changes from past behavior. This article discusses the known differences you may encounter.
Here you'll find information about events whose behaviors have changed due to multiprocess Firefox's implementation.
<option> to <select>Historically, Firefox has allowed keyboard and mouse events to bubble up from the <option> element to the parent <select> element. This behavior is inconsistent across many browsers (for example, the events do not bubble up in Chrome). For better Web compatibility (and for technical reasons), if Firefox is in multi-process mode and the <select> element is displayed as a drop-down list, these events do not bubble up. If the <select> is presented inline and it has either the multiple attribute defined or a size attribute set to more than one, these events do bubble up.
Rather than watching <option> elements for events, you should watch for change events on <select>. See bug 1090602 for details.