Logged in with firefox-esr-38.6.1, application start took about 75 secs. Very slow take-off, although once a page has loaded it's more or less sufferable. For instance navigating a page like undeadly was almost possible at 'normal' speed, but getting there will challenge your self-control. Opening a second tab is definitely NOT recommended.
I'll have to compare this again to the other firefox and the 6.0-foxes. No keyboard problems, btw.
Mozilla devs have a sense of humour: Turtle icon at the bottom with the words "Mozilla Firefox seems slow... to... start."
Also tested arora (package log attached): very scary animal, firefox is tame in comparison. Memory consumption like sth. out of a horror movie. I happened to have top running and you could literally watch it grow by the second. With each of top's 5-sec updates there was another big jump and the system soon began to crawl. I had to kill it in self defense. Otherwise it would have eaten up my computer, then myself, the house, the street...