The Mozilla Foundation just announced a new open browser plugin API effort that will also consist of players like Apple, Macromedia, Opera and Sun Microsystems. The plugin API will be based off Netscape Plugin Application Program Interface (NPAPI), which is what Mozilla browsers (including Firefox) use at the moment, and will be released under an open source license. Notice how Microsoft isn't (yet?) involved - I wonder if they'd consider this a threat (which I hope not, because it's a very smart initiative that saves everyone from doing double work trying to make their plugins cross-platform and cross-browser).
Don't confuse these plugins with extensions though - these plugins are things like the Flash plugin that allows you to play Flash and Shockwave files in your browser, and the Java plugin that lets you load applets. Maybe we'll finally have a Java plugin that isn't so iffy.
Check out the Mozilla Plugins project if you're interested in the technical details.