The fact that they're open source doesn't matter. We (humanity) have largely compatible open-source implementations of ActiveX in Wine and ReactOS, and if W3C or ISO or someone had deigned to standardise ActiveX this code would've found its way into Gecko and KHTML.

Plus, NaCl is closer than "something like" to ActiveX. What do you consider to be the salient difference between the two technologies?

The salient point is that Native Client is open source and the standard can be adopted by Firefox, Opera, and Safari if developers find that it meets their needs. Native Client is built on OpenGL (cross platform compatible) where ActiveX was developed against the Windows APIs.

Wine has been a long time in the making and certainly wasn't ready to be integrated into Mozilla for ActiveX compatibility on Mac OS, Linux, or even alternative browsers on Windows in 1996, so it's kind of a silly point to make.

Native Client is built on Pepper, a proprietary Chrome technology. The NPAPI port for other browsers was abandoned.

How do the projects being open source prevent something like ActiveX from coming to them?

It doesn't prevent a technology like ActiveX from coming to them, it prevents the CONSEQUENCES of such a technology had when it was in IE.

Ok, I still don't understand. I really got into web development around ten years ago, and have been a Firefox since back when Firefox was simply called Mozilla. So I have no experience, developer side or userland, with ActiveX. What were the consequences of ActiveX and how could open source have prevented it?

Well, ActiveX was a proprietary technology developed by Microsoft that only run on Windows, and especially only run on IE. So sites using it were only working with IE.

Webkit and Mozilla being open source means that no single company can control the projects or add a proprietary single-platform exception to them.

Now, a company could FORK Webkit/geecko and add something like ActiveX to it, but the fork wouldn't be part of webkit/geecko project anymore, and we'd still have webkit/geecko proper. For example, Google added NaCL which is something like ActiveX to Chrome, but not to Webkit itself.

Okay, so you now you have edited the source code and compiled a binary, is your recommendation that the binary be downloadable and installable by the user from the website?

Open source is one thing, but if you're not able to convince Google and Apple to ship your changes, you've hit a wall. FF implementing support for the WebKit extensions is what will break the web back into the IE-dominant era.

Note that, to get the web out of the IE-domininant era, it was first necessary for Firefox to implement IE extensions like document.all and innerHTML.

Having a competing browser is useless if it won't work with the content that people want to use.

For the example given were activeX open source then its windows-specificity would be rendered moot as some fork would displace it.

Or, if were webkit not open source then it would probably not end up being a joint apple-google production. Joe-bob's ideas for CSS that he compiles into Firefox himself may not be relevant, but eventually if FF sucks enough some fork of it will displace it as it did for Mozilla (cf. V8 displacing JavascriptCore) per some version of Kuhnian paradigm-shift.