I use version 0.8b4pre (20100331, Gecko 1.8.1) as well, and the css-editor make it more useful than SeaMonkey.
Switching directly between the source code- and wysiwyg view cause problems, most often add square characters but also disable the save button. But that occur on other os:es such as "blowing in the wind" as well. Switch between the three tabs at a slow pace from left to right, or right to left to overcome some problems.
KompoZer 0.8b4pre produce code compatible with our firefox as both was created from the same generation of the mozilla engine (as in Firefox: Mozilla/5.0 (OS/2; Warp 4.5; rv:45.0) Gecko/20100101 Firefox/45.0). The included css-editor make it quite useful compared to the newer Blue Griffon 1.1.1 from 2011 (the css editor wasn't included by default), but doesn't support new styles introduced with html5. The css editor may therefore remove code you add by hand that it (the mozilla engine) doesn't support, unless you avoid to touch the custom style.
I would have preferred the idea to have the various front ends (such as KompoZer, Firefox, SeaMonkey, Thunderbird etc.) use one (1) common mozilla engine, but other platforms however must have dictated how and what.