Pie menu's are essentially gestures with feedback, lacking the letter drawing metaphor for efficiency's sake.
ImplementationsRecent work has created a largely functional context menu replacement, RadialContext. The homepage for this effort may be accessed here or surf the iframe.
ContextTry them out at piemenus.com with MS-Internet Explorer. Don Hopkins, who introducted in 1974 the use of gestural motions to execute context-menu like functions, has released an open source IE only "behavior" (.htc) and XSL implementation. On Mac IE, you can try Andy's prototype at surfmind.
Piemenus offer a set of options distributed equidistant from the source event (eg. user click). This is superior to the dropdown menu for speed of execution and learnability. There's a lot of work on the mathematical underpinnings of this benefit, expressed by Fitt's Law.
Development OptionsThere are two potential areas of implementation in Mozilla. The first is XUL, the interface toolkit of Mozilla's UI. This could provide a methodology both for supporting Mozilla UI development as well as incoporation by page authors.
A more general web deployment approach would be as a .js library, using DHTML to craft the menus. This could be conditionally switched for the IE version for deployment.