Since the only respondent to my Mozilla Firebird Support forum suggested that the answer to this site’s display problem with Firebird might be resolved by the fix to Bug 215857, I installed the latest Firebird nightly build this morning. Sadly, as far as I can tell, there is no change. I’ve relayed this back to the forum and hopefully will get some useful information back. If any of you are reading this in Firebird, sorry for the annoyance.
Later: My post to the forum got a fast response from Chris Neale. His answer, to add a width: 98% to one of the main styles, was apparently just what was needed. The Firebird issue has been handled. Nice!