Whoa! Same FireFox add-in work on Windows & Mac?

Most Firefox add-ons (and most of Firefox, for that matter) are nothing more than Javascript + XUL (the UI markup language used in Firefox)...  of course they're cross-platform.  Add-ons requiring more functionality (like the 3D stuff CoolIris does) will occasionally be platform dependent (as they use native code for some stuff), but those should become less common as Firefox introduces new features into the browser (like Canvas, native video support, etc.)

You don't see a lot of full-blown applications written using Firefox's engine because the tools to make it easy are relatively new (for example, Firefox didn't package XULRunner, the component that lets you run stand-alone XUL applications, with the browser until 3.0).  Nevertheless, one of the apps we have here @ the research group I'm working with is written entirely using Javascript + XUL + Flash.  Some of my coworkers were somewhat surprised a few weeks ago when I got it running on my Mac (it was developed on Windows).

None of the others are built on a platform-independent UI framework...  it would be far more difficult for Chrome or Safari to implement cross-platform extensions with as much versatility as Firefox has, because they'd essentially have to rewrite from scratch some kind of complete UI framework, accessible through Javascript and/or some kind of markup language, then have to implement all the hooks to give these extensions somewhere to live (Firefox gets these for free in most cases because of the way XUL works).