Also, it looks like the framerate in Internet Explorer, and Safari, and the Mac version of Firefox all work fine. Somehow the framerate in the PC version of Firefox is consistently very bad...
Are these sorts of inconsistencies among Flash Players for different browsers common? Are they avoidable with certain best practices?
Welcome to the fun!
Unfortunately this is the way it is. It is really impossible to say anything certain as the OS / Browser type (FF, IE etc) / Browser version (2,3,4) / Plugin version can all form a pretty deadly mix of variables.
My experience has been that on a fast mac and older pc... previewing swfs in CS4 has been pretty crumby as opposed to most browser plugins. I get a much better experience in a browser.
Some versions of FF can get bogged down by Flash. Although I shun IE for web browsing it usually handles flash the best.
I think most people will tell you that Safari + Flash can really get a mac's fans going with some large cpu spikes.
Are you using fonts in the map? Are they set to "anit-alias for readability"? That might be something to look into.
If you post a link to your work, maybe some people can chime in with their experience and we can narrow it down to a certain version of FF or see if anything jumps out as a "trouble area" in your execution.