It's in Nightly. I've seen it a few times. It appears as a bar across the bottom of your screen telling you x addon may be causing this page to load slowly. From there you have the option to disable the addon, ignore the message, or ignore permanently. I saw it for uBlock, HTTPSEverywhere, and reddit enhancement suite, and amazonsmile redirect. In each scenario I felt it was well worth the tradeoff to run a little more slowly.

Worth noting that it's set to be hypersensitive right now for testing. Don't trust it for much of anything yet.

Good to know, I was starting to think either noscript/ublock were badly written, badly integrated or that Firefox had deep issues.

Thanks. I found it. http://www.ghacks.net/2015/02/18/firefox-38-notifies-you-abo...

It's in current nightly builds. You get a status bar telling you that such add-on is taking to long when you load a page.