• https://www.userchrome.org/how-create-userchrome-css.html

statuspanel[type="status"] { display: none !important; }

Where can I find the docs on this sort of option?

Use Browser Toolbox to determine element IDs and go from there.

• https://developer.mozilla.org/en-US/docs/Tools/Browser_Toolbox

Note that you can use more specific selectors to hide only some of these messages.

statuspanel[type="status"] .statuspanel-label[value^="Look"]
statuspanel[type="status"] .statuspanel-label[value^="Connect"]
statuspanel[type="status"] .statuspanel-label[value^="Wait"]
statuspanel[type="status"] .statuspanel-label[value^="Send"]
statuspanel[type="status"] .statuspanel-label[value^="Transfer"]
statuspanel[type="status"] .statuspanel-label[value*="TLS handshake"]
statuspanel[type="status"] .statuspanel-label[value*="FTP transaction"]

Thanks to Moses and cor-el for the help. Solved something that was really annoying me without having to use an add-on.

Perfect!

The solution in this thread is no longer working for me in Firefox Nightly. I have created a separate thread for a new one:

https://support.mozilla.org/en-US/questions/1213021

https://www.reddit.com/r/firefox/comments/8ayc2d/old_userchromecss_from_ff57_not_working_on_ff/

Note that it is just a matter of changing to IDs for Firefox 61.

#statuspanel[type="overLink"] #statuspanel-label,
#statuspanel[type="status"] #statuspanel-label[value^="Look"],
#statuspanel[type="status"] #statuspanel-label[value^="Connect"],
#statuspanel[type="status"] #statuspanel-label[value^="Send"],
#statuspanel[type="status"] #statuspanel-label[value^="Transfer"],
#statuspanel[type="status"] #statuspanel-label[value^="Read"],
#statuspanel[type="status"] #statuspanel-label[value^="Wrote"],
#statuspanel[type="status"] #statuspanel-label[value^="Wait"],
#statuspanel[type="status"] #statuspanel-label[value*="TLS handshake"],
#statuspanel[type="status"] #statuspanel-label[value*="FTP transaction"] {
 display:none!important;
}