UX: Indicate current download progress
While training journalists on the use of SecureDrop, which heavily uses Tor Browser as its primary interface, users consistently confuse the arrow flash animation over the hamburger menu in the Tor Browser as a sign that the download they started has completed, rather than simply having been started. They panic when a partial file they prematurely copied fails to open, and someone ends up having to make a house call to let them know the file wasn't finished downloading.
Vanilla Firefox makes a nominal effort to address this with an arrow button appearing in the toolbar aside from the hamburger menu which indicates download progress. Including this existing Firefox feature into the Tor Browser would be a useful first step, however, other browsers have solved the potential for this kind of user confusion by showing individual download progress in a new toolbar. Copying what Brave Browser does (see image here: https://imgur.com/LZU7hAo), for example, would much more effectively help prevent confusion on the user's end and should be implemented in Tor Browser, then pushed upstream to Firefox.