Due to API limitation native sidebar cannot open or close itself. Details here: https://developer.mozilla.org/en-US/Add-ons/WebExtensions/API/sidebarAction/open
But you know, its a weird request anyway. =)

Edit: filenames should be crossed only when file deleted not downloading, its a Firefox specific bug.

"download speed or rest time of downloads".
Yes it is possible. New features would be implemented in 0.4 branch, 0.3 now is only for bugfixes. So it takes week or two or maybe more. =)