Firefox told me to come here after my review and I must say damb their good but there's one problem.
I have seen loads of requests for this but none have actually been fulfilled.
Default desktop site mode. I think it's beneficial to have a mode where when you select desktop mode on a certain tab it will always stay in desktop mode until you tell it otherwise. So when you enter and leave Firefox it that same tab will always be in desktop mode.
Hope this helps not only myself but others too.