Chrome has a significantly more restricted extension API than Firefox; it mainly just consists of content scripts (with background pages) and a couple of hooks for context menus and popups and such. As such, it seems to me that a very significant subset of the Chrome API could be implemented in Firefox, either as a Firefox extension or a separate "extension compiler" like there exists for Greasemonkey scripts (which, incidentally, are another example of something that works exactly the same way as Chrome content scripts). If implemented, this would allow most Chrome extensions to install and run in Firefox.
I Googled around a bit and found several mentions of a very old attempt at this, but all of the links to the actual Mozilla repository for it are now dead. Even if they weren't, I'd be very surprised if this early effort still worked with either Firefox 4 or the latest changes to the Chrome extension API.
So my question is, does anybody know what the current state of the art is with regards to Chrome Extension API compatibility layers in Firefox? Have there been any extensions or extension compilers attempting to address the issues I brought up?
Best Answer
If there is such a beast, even Mozilla developers don't know about it!
Many Chrome "extensions" are just Greasemonkey scripts, and these can be used with the Greasemonkey extension or compiler on Firefox with no changes.
Unfortunately, extensions that use other APIs must be ported. If you're looking to port a Chrome extension to Firefox, or planning to write one for both, take a look at Mozilla's new Jetpack extension API, which was designed to work in a manner similar to the way Chrome's extensions work, but still offer the vastly increased access to the browser the original Firefox API is known for.