The beta version is of course rough around the edges, and one rough spot is the fact that often when editing css properties of an element on an html page, the entire style definition would simply disappear from the console.
Fortunately, as of today, the firebug-1.5X.0a07.xpi release fixes the problem. Click the link for 1.5X0a07.xpi within Firefox to update. I haven’t found any serious problems with the alpha version yet.
As a side note, if any of you are Safari users or are just testing your sites out for safari, make sure you enable the development menu (Preferences > Advanced > Enable Develop Menu) and then you can use the command-option-i to show a Firebug-like inspector. It’s growing on me each time I use it.