Posted: Mon Jun 17, 2013 11:31 am
Sorry that I didn't check Firefox, I didn't think Firefox would handle that bit differently than Chrome. I'll make a note to fix that later.
I looked into this but I'm sorry to say this isn't fixable. The way Firefox interacts with the taskbar involves putting data into the Windows Explorer process. Now since you're running as full Admin, your Windows Explorer is only accessible to administrators. At the same time, you're using Drop Rights which means the process in the sandbox does not have administrator privileges. So it can't put data into the Windows Explorer process.
You can prove this to yourself by using Process Explorer, double click to display properties for explorer.exe, view Security tab, click the Permissions button on the bottom (just above Cancel), then add your user account and give it full access. This way you're making it possible to put data into Windows Explorer for processes that run with your user account but don't have admin privileges. And then your rights-dropped sandboxed Firefox will be able to un-hide the taskbar (as a result of being allowed to put data into Windows Explorer).
If you're saying this worked in version 3.76 then that's actually not something to strive for, and while the new behavior may be less convenient for you, it is nevertheless the correct behavior.