The goal of the project is to run web content in a separate process from Firefox itself. The two major advantages of this model are security and performance. Security would improve because the content processes could be sandboxed (although sandboxing the content processes is a separate project from Electrolysis). Performance would improve because the browser UI would not be affected by poor performance of content code (be it layout or JavaScript). Also, content processes could be isolated from each other, which would have similar security and performance benefits.
Although the Gecko platform supports multiple processes, the Firefox frontend is not designed to use them. Work to make the frontend (including addons) support multiple processes was begun in early 2013. The project roadmap has more details.
It appears to be better than it was a month or so back. There was a significant lock-up when restoring a session with hundreds (to thousands) of tabs ---- 5-10 minutes for FF to finish parsing whatever the hell it is parsing, before it would be useable. Even with the Option/General: Tabs [x] Don't load tabs until selected.
LastPass is acting funky with the most recent Nightly, first time for that, imagine it will be fixed up in the next day or so.
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