Have you come across Yahoo!’s YSlow plug-in for Firebug? It scores your page against their “Best Practices for Speeding Up Your Web Site”, and is handy for looking at the page’s external resources and relevant HTTP headers.
If you have used YSlow before you might be interested in the upcoming YSlow 2.0. I just came across a slideshow while looking for URLs for my comment above. The extensibility looks quite interesting.
Firefox plugin API really needs an overhaul. It shouldn't be assumed that everything breaks on every point release. Firefox needs to come up with an API going forward and stay backwards compatible till the next full version number.
I think Chrome is going to lead the way here. Their plugin strategy seems to be based off fixing the mis-steps of firefox.
I just use extensions.checkCompatibility=False in about:config and most extensions work just fine.
Also, they've changed addons.mozilla.org where most addon authors just have to flip a dropdown box to move support to a new version-- no new upload required. Many authors don't know that, so they wait until the full release to change their support.
FF 3.1 should break very few extensions. Almost none of mine died, though Firebug is particularly sensitive. But I used Firebug 1.3.0 betas just fine for a while on FF 3.1. (My new laptop isn't using FF3.1 at this moment, though.)
sorry to say this, but I think their API is great. They are innovating, and innovating requires breaking something. If the devs can't keep up, they shouldn't be creating extensions in the first place. Same could be said of iPhone development as well... it just so happens that the cool platforms tend to have more underlying changes than static platforms aka windows. (and even under windows, development can be shaky if you're doing stuff in .net)
EDIT: I'd also like to add that this version of Firebug actually does work in FF3.1. It's only that in their install.rdf file, they specified 3.0.*. I changed it to 3.1b2pre and re-built the extension and it seems to work great under Minefield. I think they put the version restriction to prevent people from reporting bugs for the still beta FF3.1.
http://remi.org/2009/01/06/using-firebug-to-debug-unobtrusiv...
Now you don't have to guess what events were bound to which elements!