Same problem as with 6.0.15 for me. The Evernote button does show much faster, but the browser still stays unresponsive (one CPU core constanlty at 100%) and the memory usage skyrockets. Additionally, as before, using about:memory - the per tab window/event target/event listener are gone - but enabling Evernote stiil causes a massive 5GB heap allocation.
Anonimized memory reports - before enabling Evernote Web Clipper, After enabling Evernote Web Clipper.
Firefox 37.0.2, Linux x64, Evernote Web Clipper 6.0.17, no other extensions enabled.
As I use Evernote extensively in my day to day workflow - missing it is a major pain, so I'll gladly help as much as I can fixing this problem