If you're about to install a Firefox extension, you're out of luck: The Mozilla add-on site is down.  Of course sometimes it's not the destination, but your connection - so how can you know?   A very appropriately named site:  downforeveryoneorjustme comes to your rescue:


The site will quickly tell you if you have a problem, or, as is the case now:

 Huh? doesn't look like a site on the interwho.

(As good a name they picked, it's longish, you might want to bookmark it)