OK, I finally found some time to address many, but not all of the annoying and lingering problems using FlexWiki with Firefox. This came partly down to CSS work and partly down to how script and forms all fit together in the initial implementation of the topic editing page. I've gotten the two major Firefox problems fixed now (font sizes and the edit page working). There are a few other minor things I got working, too.
There are two features that you get in Internet Explorer, but not in Firefox yet: TopicTips and the TopicBar. These are nice features of FlexWiki, but not essential to its operation. There's no principled technical reason why these don't work yet; just time :-)
This is running live on FlexWiki.com and will be available in build 1593 (available at flexwiki.com within a few days).