Mozilla has removed XUL <textbox>, although only partially in the ESR68 timeframe (see legacy/trac#32154 where we had to replace a multiline usage with <html:textarea>). We need to finish the job and replace our remaining uses of <textbox> with <html:input>. While we are at it, we should double check that we do not use <textbox> outside of Tor Launcher, either in the Torbutton code or in a Firefox patch.