If you switched to Daylight Savings time in most of the US/Canada the change should be automatic (FALL back).

Change your settings by right-click on the clock in lower right corner, and choose "Adjust date/time" (Windows 7)

Under "Change time zone, indicate whether you change time for Daylight savings time.

I looked all over Yahoo! mail settings (classic mail and new mail) and didn't see anything about time zone. So it probably is an issue with how Firefox is reading your Windows system clock.

You can see the current time and time zone if you paste this code in the command line of the Web Console (Web Developer > Web Console; Ctrl+Shift+K). Paste each of these one at the time in the command line and press Enter to see the result.

• "3-bar" menu button or Tools -> Web Developer

• (new Date());
• (new Date()).toString();
• (new Date()).toLocaleString("en-US",{day:"numeric",month:"long",year:"numeric",hour:"numeric",minute:"numeric",second:"numeric",hour12:false,timeZoneName:"long"});

See also:

• https://developer.mozilla.org/en/Tools/Web_Console