I'm running Windows, and I get that blank page for 500 errors, i.e., when I make a dumb error in my PHP. I'm not aware of any built-in feature to give a narrative explanation.

There have been some bugs filed, but the localization challenge and various other changes seem to have hung them up. (I know I'm generalizing here, but I read a bunch of bugs and didn't keep track of the details.)

I didn't see any add-ons that addressed issues other than 404, but there might be one not on the add-ons site.

It seems like a logical subject for a Firebug add-on. Not sure how to search for those.

Hey jscher2000, my main problem is that during development (where I have to use a load balancer to simulate the prod environment) it's impossible to tell at a casual glance what was the culprit - Did I screw up my web app, is the apache instance on the server dead or dying, maybe tomcat, or the ELB may have dropped the instance from rotation, etc.

Every other browser communicates this nicely - Error 503 page displays in Chrome & IE, which indicates that the ELB has dropped the instance. No troubleshooting or investigation required.

Using the FireBug net panel will let you see that the response is a 503, but the fact it's necessary is a bit disappointing, especially seeing as its limited solely to Firefox. And of course the Firebug panel can start to impact performance on heavy websites.

I'll continue using Firebug in the meantime, thank you for taking the time to answer!

Edit: This doesn't seem to be working when installed this way. It only seems to work in Test mode with the Add-on Builder Helper extension. Annoying.

Edit #2: If I disable the extension and then re-enable it, it works again once for a given URL. Not sure why it works differently than the test version.

I created an add-on to pop up status 500 or higher in an alert. (I tried to inject it into the page, but unfortunately I didn't get the sequence right and my change was overwritten so I went with the alert.) If you want to take it for a spin, you can get it here:

https://builder.addons.mozilla.org/user/jscher2000/

Click the View button, then the download button (the first arrow button, next to the eye), and save to a convenient location. You can install it by dragging it onto an open tab.