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So far after using the Firefox nightly for a few weeks I have to say it is excellent.
No slowdowns on any content including flash/java/vids.
"Essential" add-ons all available. Adblock/Flashblock/Noscript/Downthemall/FVD+Sync/WoT
1 amazing add on I found is Scrollbar anywhere. With a rightclick and drag up or down it scrolls the page as if you were using the scrollbar. right click context menu still available.
I was disenthralled by Firefox over the last couple years over bugginess, performance problems, and illogical behavior changes. But with FF 29 it seems to be all fixed. Give it a try and post back your thoughts if you can.
.......no I dont work for Mozilla. Just posting on a topic that I care about which is finding a browser that does everything right.