Really? I haven't heard a single thing about Electrolysis in a long time, except I remember reading somewhere recently that it was abandoned. I'm happy to hear that Firefox OS is using it; still, keeping legacy add-ons working seems like a bad reason to stop pursuing such an important security feature on the desktop.
I was employed at Mozilla when the pronouncement to "suspend" work on Electrolysis came from on high.
I think addons are a small part of the picture.
The big deal is there are only so many Firefox developers, and Electrolysis was turning into a real sinkhole of time with no end in sight. There were a lot better ways to make Firefox a better browser in the meantime, and those things are being done.
Firefox on Android used to use Electrolysis until it switched to the current Java front-end. And FirefoxOS is using it for sure: process per app, so that apps can be easily killed in low-memory conditions.
As for "legacy add-ons", that would be "every single add-on". Not to mention that the desktop Firefox UI itself would have to be heavily rewritten as well. The judgement, as mcpherinm says, was that the cost was too high for the possible gains. I agree that process isolation is good for security, but there are various other security mitigation strategies that can be used that had a higher bang for the buck at the time.
