There were beginnings of talks to get cross-vendor consensus on Pepper (which would be a prerequisite to any standardization attempt of PNaCl) in 2010: https://mail.mozilla.org/pipermail/plugin-futures/2010-April...

The general consensus among non-Google browser vendors was that just using the existing browser APIs was a more desirable approach than Pepper. Since then, Pepper has remained a Chrome-specific technology.

Could Google add NaCl support to Firefox with an add-on?

They could, but why would they?

To help developers?

Judging from their FAQ [0] on it, it doesn't seem like they are generally opposed to the idea, just see it as premature at this time.

[0] https://developer.chrome.com/native-client/faq