I've noticed this in both Firefox 9.0.1 (& 10.0) stable and Nightly 13.0a1 64bit (using Greasemonkey 0.9.15)
Disabling GreaseMonkey (& restarting Firefox) makes update checking to finish as expected all the times.
Re-enabling it, and the problem reappears.
If it helps any, I also these many errors in Error Console:
described in here:
Warning: WARN addons.manager: Exception calling callback: ReferenceError: AddonManagerPrivate is not defined
Source File: resource://greasemonkey/script.js
versioncheck.addons.mozilla.org : server does not support RFC 5746, see CVE-2009-3555
I've also made this in bugzilla:
When manually searching for add-on updates, sometimes the "Updating add-ons" message never finishes
PS. Yes, I've read 0.9.12.beta2 affecting "Check for updates"
Confirmed. This only happens when there is a new update found.
The above fix needs to be amended to not break Firefox 3.
Import AddonManager module for AddonManagerPrivate reference.