Write an extension to leverage the existing spellcheck code in Mozilla and add the ability to highlight spelling mistakes for a given web page (i.e., vs. a textbox).
== Modify Firefox to throttle Plugins when Inactive ==
Modify the browser so that it throttles plugins back when the user/browser/system is inactive. Consider, for example, [http://www.kaourantin.net/2006/05/frame-rates-in-flash-player.html work] that has been done in Safari. You could consider doing this via the IdleService, with an observer set to go off at an appropriate interval. You should also consider how to deal with the case of videos (YouTube). Perhaps keep a white list of movie sites, or other plugin-dependent content that shouldn't be throttled.
Resources: gluon
== Modify Firefox to handle files downloaded to Temp more appropriately ==
