If you are using NoScript or some similar extension that limits JavaScript, you must (temporarily or permanently) allow mozilla.org for that service to work with https://.

Thanks for your reply. I had already disabled AdBlock Plus and set NoScript to allowed the update page to use JavaScript and that didn't change anything. Also, other web pages using verify and https:// work in FireFox including this support page.

I just tried it again this time with NoScript set to allowing Scripts Globally (which I had tried before) and it still didn't work. So I disallowed global scripts and then disallowed webtrendslive.com in NoScript and when the page refreshed it worked. I exited FireFox, restarted and tried the plugin update again and it failed. So this time I allowed webtrendslive.com and again after the page refreshed it worked.

I restarted FireFox again and after the plugin update check failed I tried clicking on the Retry button (which had always failed before) and it started to get status but failed so I clicked on the Retry button again and it worked. I have also tried just refreshing/reloading the page after it fails the plugin update check but that has never worked.

It seemed like it always fails the first time I try using the plugin update check and I can make it work using one of the methods above, after that it works fine as long as Firefox is not exited and restarted. I say it "seemed like" because I just tried restarting Firefox and using the plugin update check and this time it worked the first time. So it might have something to do with my very slow Internet speed (dial-up) and how busy the FireFox server is at the time but if that's the case why does it work every time using http:// instead of https:// with the verify?

Do you have that problem when you go to this page directly?
http://www.mozilla.com/en-US/plugincheck/

That page loads a bit faster for me than the https page does from the Addons > Plugins window using the Find Updates button - and I'm on a 3MB cable connection.

No, no problem when I go directly, that's what I'm referring to when I say I use the http:// instead of the https://, and the direct page also loads way faster for me too. Is there a way to disable the secure address (https://) when using the Find Updates option so that it doesn't use encryption?