After investigation, the problem is not jQuery but jQuery UI. The (rather old) version embeded in LimeSurvey doesn't work with Firefox 12, after a manual update of the files to the latest version :

It works !!!

Fix committed to master branch: http://bugs.limesurvey.org/plugin.php?page=Source/view&id=8235

Fix committed to Yii branch: http://bugs.limesurvey.org/plugin.php?page=Source/view&id=8236

Thank you very much for the analysis!

With the news jquery-ui.js, the admin works, but I've a problem on a survey, I've put a "yes / no" question, and the date picker is only shown if the user select "yes".

But when the datepicker is hidden, the div "ui-datepicker-div" is displayed on the bottom left of the webpage (just a blue line). If I try with the jquery-ui.css from jqueryui site, it works, but the admin backend is quite ugly.

(I've joined a screenshot)

What is the procedure for getting a version with the scroll-bar fix?

(Clicking 'check now' for updates in the admin interface shows no new updates available).

Even if there is some collateral damage in the fix, I'd like to try it,
because there is no way to downgrade firefox on Ubuntu (except, strangely, to the much-older ff 7.0.1).

Thanks

Fix committed to master branch: http://bugs.limesurvey.org/plugin.php?page=Source/view&id=8275

Fix committed to Yii branch: http://bugs.limesurvey.org/plugin.php?page=Source/view&id=8277

New 1.92+ build released