Right now? Playing with Firefox Nightly (which will be Firefox 35), with experimental e10s support turned on. This is similar to what Google Chrome does in having each tab and extension in a separate process, but Mozilla is using a different approach using threads instead of full processes. It's a good idea and I'm all in favor, but right now it breaks about half the extensions I have installed. I'm going to have to set up a separate profile for testing e10s where that's turned on, and use one where it isn't for production browsing.
Fun, for suitable values of the term.
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Dennis