I think developers, and users, define "speed" differently.
(this comment typed late at night, so forgive me if it's less than coherent)
It seems to me that when the Firefox devs talk about "making FF faster" they mean: "hey, look, we completely re-wrote the engine and now we score 1.2% faster on Sunspider!!1!!"
Whereas when users talk about "making FF faster" they mean: "I really hate the way it drags like a slug through treacle as I scroll down certain web-pages."
To quote a specific example: I use FF on Windows 7 & 10, Mint Linux, and OSX 10.9.5. For 90% of the sites I visit, it's perfectly fast enough. But there's a couple - Jalopnik.com and the Telegraph.co.uk main page - that bring the browser to a crawl (for reference, I just loaded the latter on my Mac. Firefox's CPU load in Activity Monitor went from 4% to 105% as the page loaded, then settled down. When I scrolled down to the "Gallery" section of the main page, CPU usage went through the roof - 120%+, and the whole page juddered like I was running a 486). And that's with NoScript, cookie blocking, and so on. I couldn't give a fig whether the browser beats Chrome in <benchmarkX>.
As long as devs & users continue to have completely ideas of what "more speed" means, and the devs continue to arse around with the UI for no good reason, FF will continue to haemorrage users.