This is an incredibly useful and well-designed extension overall, but it doesn't play nice with some websites. For example, blogs.worldbank.org/ refuses to load if I have FoxClocks enabled—I've tested it a bunch of times and FoxClocks is definitely the culprit (I'd be interested to know if others have this issue or not).

So I keep FoxClocks on for the most part, but if I'm having trouble with a site, disabling the add-on is one of the first things I try. Just an FYI for other (potential) users.