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Prism and Extensions

One of the most frequent questions I get about Prism is how to make it work with such-and-such beloved Firefox extension. One thing is certain, to make an extension work with Prism you have to add Prism’s application ID to the extension’s install.rdf file, as described here. The ID for Prism is prism@developer.mozilla.org. The easiest way to install the modified extension is to enable the status bar by setting status=true in your app’s webapp.ini file. You can then select Tools/Add-ons… in the “gear” menu in the status bar and click the Install… button in the dialog. Open the XPI file of your extension to install it.

However, that probably isn’t all there is to it. Many (most?) extensions include some kind of overlay that modifies the main browser chrome window. This file is called browser.xul in Firefox, but webrunner.xul in Prism. So you’ll also need to go into the chrome.manifest file in the extension XPI and change the overlay line. Say it reads:

overlay chrome://browser/content/browser.xul  chrome://extension/content/overlay.xul

You’ll want to change that to:

overlay chrome://browser/content/webrunner.xul  chrome://extension/content/overlay.xul

I sound like a ginsu knife salesman, I know, but that’s still not all. If the overlay modifies, say, the browser’s popup menu, in Firefox it would want to modify a <popup> with the ID “contentAreaPopupMenu” whereas in Prism it’s a <menupopup> with the ID “popup_content”. I don’t have a good explanation for why the IDs differ. I can only say that this decision predates my involvement in the Prism project. You can compare and contrast the IDs used for various browser chrome elements by looking at the code for browser.xul and webrunner.xul.

This should be enough for the brave of heart to hack a lot of Firefox extensions to work in Prism. But it’s hardly ideal. There are a few ways I can imagine to improve the situation:

• Harmonize the IDs of chrome elements shared by Prism and Firefox. At least this would make a lot of existing overlays work in Prism without modification.
• Rename webbrowser.xul to browser.xul. This would remove the need to modify the extension’s chrome.manifest in most cases.
• Implement an intermediate browser layer that can be used by Firefox, Prism and other XULRunner-based browsers. Right now, a lot of browser code in the Mozilla tree is under the toolkit/ directory and can be used by any XULRunner app (e.g. the download manager, extension manager, etc.). But some code is in the browser/ directory, which is part of Firefox, not XULRunner. It might make sense to factor out the code in browser/ that would be useful for other web browsers. Among other things, this would probably mean splitting browser.xul into two files, one used by Firefox and the other in the shared layer. Firefox extensions would mainly hook into the shared layer so they would work without modification in other XULRunner-based browser apps.
• Base Prism on Firefox. This is an idea that we discussed ages ago but somehow fell by the wayside. In a nutshell, Prism would be Firefox, but with most of the chrome hidden (e.g. toolbars). This would mean that pretty much any Firefox extension would work out-of-the-box in Prism. It would also solve many other Prism issues (e.g. people are always complaining that Prism lacks some feature — like Quick Find — that then has to be added to webrunner.xul by hand). The biggest challenge would be to hide the requisite browser chrome. I’ve yet to determine to what extent this might pose problems, but Daniel Glazman’s FullerScreen might provide some useful hints.

The idea of an intermediate layer is appealing from a software engineering standpoint, but doesn’t strike me as particularly realistic because of the changes it would require in Firefox code. I’m leaning towards basing Prism on Firefox (as we originally planned to do months ago).