Without going back to find the post -- another feature that doesn't appear in Firefox while using "compact" Zone mode -- my memory tells me:

Shortly after I implemented Firefox 0.8, and posted here about incompatibilities, Robert explained that the "Preview" function exploits something unique to IE. In other words, I don't think any amount of tinkering with FF will get it to show up.

I've just been cruising the FF Extensions page at Mozilla. There's some interesting stuff in the Developer Tools category that can display the code for a page (or frame) in easier-to-read format than the basic right-click "View Source" function... but I have no idea if it could be reverse-engineered into a rudimentary HMTL composer/previewer.

But -- there's good news on the horizon! I hope Robert doesn't mind me reproducing a paragraph of his from a recent email exchange we had:

Don't try to sell me on FireFox, you're preaching to the choir. I like it as
a user and as a professional. My new doctrine of Web design is to make a
page work first in FireFox (Mozilla et al), then make the changes necessary
to make it display well in IE. The next generation of the Zone [...]
will be designed from the start to work in FireFox. FireFox will be the
native browser.

Meanwhile... on the rare occasion when I'm making a complicated post (lots of italic, bold, blockquotes and/or links), I use my HTML editing ap*, save the page, and preview it that way.

[* - AceHTML Freeware 5.0 from VisiComm ... sadly, discontinued, or I'd provide a link. There must be others out there...]