I've got exactly the same issue reported here: Plugin won't enable
I've created a really simple plugin that works fine on my local environment but won't enable on my remote dev server. I've followed the advice in that Question and checked all the file and class names but with no success.
There doesn't seem to be any indication of problems in the logs either. I'm stumped. =(
The directory structure is simply just: /plugins/firstimage/twigextensions