So yes, UserOptions.css is an option, but since edits require Firefox restarts I picked "Generic CSS Loader 2" add-on for Firefox because it can show me if I succeed and is lightweight (less than 10kb) especially compared to Stylish.
It took me ages to find out why my CSS is not applied. It requires !important to be added.
Display the "Download in other formats:" along with its links also left, but not leftmost and only on tickets.
@-moz-document url-prefix("https://trac.torproject.org/projects/tor/ticket/"){#altlinks {text-align: left !important;margin-left: 6%;}}
Note that margin-left works without !important because it doesn't need to be overridden. Also note that this should work for https:// only, which is no issue since it gets shipped over https anyway.
The quotation marks (") around the domain and URL seem to be not required. You can play around with the percentages or replace them with pixels (px).
@-moz-document domain applies everything to a given domain.
@-moz-document URL applies everything to specific URL.
@-moz-document URL-prefix applies everything to URLs that begin with what you specify.
I'm not going to close this ticket, yet. I'm going to wait for some input e.g. for opening a ticket in Trac's Trac (if this is a something that affects Trac as product) or if this can be an optional thing in Tor's Trac settings, or whatever.