It's usually less so things breaking and rather just being slow. For example, if you use NoScript, it often takes really long to load up a page, because that's

It's usually less so things breaking and rather just being slow. For example, if you use NoScript, it often takes really long to load up a page, because that's when NoScript becomes active, and because NoScript isn't yet properly e10s-compatible, it just forces Firefox to do things in a single-process way. But that actually makes things slower than when Firefox is running without e10s. about:performance is a rather good indicator, if an add-on is causing slow-downs.