[Jan 12, 2012] A note form Google's help Desk regradign their new Page Loading Speed Tests:
Starting around Nov 16, 2011, many sites saw increased Avg. Page Load Times. These inflated metrics are not a real speed issue for the sites, but a result of bugs in Firefox’s Navigation Timing implementation, specifically how the start of navigation is calculated. As a result of this, pages viewed in Firefox are showing higher values for Avg. Redirection Time thus Avg. Page Load Time. This is expected to be fixed as part of Firefox 9 release. All data for other technical metrics and browsers are not affected.
If your site has higher page load times during this period, you can determine if this is due to Firefox by checking the data along the Browser dimension.