Thunderbird 24.5.0, Add-On Lightning 2.6.5 . Calendar defaults to 60 min. "event length". Can't increase duration manually in "edit event" dialog, nor "options"

My computer is an ACER Aspire with Windows 7, 64 bit, i3-370M processor. I have used Thunderbird and the Lightening Add-on for years now and love it! The above problem (Event duration cannot be adjusted either shorter or longer than 60 minutes.) has started happening recently (I am assuming after a recent automatic update to one or both programs). To resolve the problem I attempted: 1. To change the "end" time in the "event edit" dialog box. When I changed the "end time" from 9 AM to 12 PM, the "start time" moved up from 8 AM to 11 AM, Ie. maintaining the 60 minute default. 2. I then tried to change the default event duration from 60 minutes to 240 minutes in "Options". the Dialog box accepted the change, I saved; rebooted; checked options to ensure it had retained the duration change (it had), but the "event edit" dialog still defaulted to 60 minutes duration and could not be manually increased or decreased. Can you help?