The faster JS is attractive if you use a lot of JS heavy sites,. I do not - in fact I mostly browse with JS off. Start up time is a once a day issue, it is not significant given all the other apps usually in my session, and will be even less so once I solve an issue with suspending to RAM.

I would like process per tab, but I like my Firefox extensions more (Scrapbook, Clear fields, No Script, Its All Text, Errorzilla, Download Helper, Add to Search Bar, Web Developer, No Squint, Search Status).