I have added concurrency and jpeg support for this amazing testing tool 🙂
If you came across this issue like me here is how to update slimerjs:
npm install -g slimerjs
mv slimer slimerjs-bak
mv slimerjs-0.10.0/ slimer
with new v10.0 you need to update your firefox version too. you can choose a version between 42 and 46.