FYI - as a workaround, redownload the JSON files (http://hudson-ci.org/updates/hudson.tasks.Maven.MavenInstaller.json and the other tools, or http://hudson-ci.org/update-center.json), and remove everything but the actual {} JSON object. Save the file(s) (removing the .json file extension for the tools, though not for update-center.json) and you should be able to function.

I added an onException handler in downloadService.post and found it was getting an exception in nslXMLHttpRequest.setRequestHeader (with mac firefox 3.5).. seems the requestHeaders parameter to Ajax.Request is required.. adding requestHeaders:[] seems to resolve the problem.

I'll commit a little fix to avoid js error in firefox after successful update.. kohsuke will commit main fix for the issue (crumb object adds bad header with empty value). This will resolve both update of update center data, and refresh of plugin/hudson upgrade status on the /updateCenter/ page.

(the requestHeaders:[] workaround above was just a bandaid for that particular case to overwrite the bad/empty header added elsewhere)

The root cause was in our modification to Ajax.Request, so this pretty much affects all AJAX activities in Hudson.

ok, I downgrade because I fixed files this morning and probably the update center automatically updated them and broke them again

Code changed in hudson
User: : kohsuke
Path:
branches/rc/test/src/main/java/org/jvnet/hudson/test/HudsonTestCase.java
branches/rc/test/src/test/java/hudson/model/DownloadServiceTest.java
branches/rc/test/src/test/resources/hudson/model/DownloadServiceTest/testPost.jelly
branches/rc/war/resources/scripts/hudson-behavior.js
trunk/www/changelog.html
http://hudson-ci.org/commit/27147
Log:
[FIXED HUDSON-5536] in 1.345 and added a test case.

Code changed in hudson
User: : mindless
Path:
trunk/hudson/main/core/src/main/java/hudson/model/DownloadService.java
trunk/hudson/main/core/src/main/java/hudson/model/UpdateSite.java
http://hudson-ci.org/commit/27145
Log:
HUDSON-5536 set text/plain content type in doPostBack response so browser won't
try to parse response (resulting in "no element found" in firefox error console).
Not the fix for 5536 itself, but nice to avoid js error after successful update.