It is easy to import any bookmark file from All Bookmarks(which gives Library| Import and Backup button.
As for the Portable download, there was no recommended download when I looked up, although one could still get the file, and, potentially make a mess. Again, the Portable site is not a Mozilla site.
I, personally, would be leery about depending, only, on a Portable app., and worse yet, a non final version, so I am carrying on the hard drive both the final version, which is installed, and the Portable one, which is not installed. By the way, did you read the [url=http://portableapps.com/news/2011-01-25_-_firefox_portable_4.0_beta_10_released] the page about b10? I can see, very clearly in bold letters that the download is not available
I have not tried any beta, but if I would, I would create a separate folder to work on it. You can copy the contents of the previous version, if you wish. And, see what is compatible.
As for the places.sqlite, read this article
. My FF 4 b10 has both the bookmarks.html and the .json files in the bookmarksbackup folder.
UserAgent: Mozilla/5.0 (Windows NT 5.1; rv:2.0b10) Gecko/20100101 Firefox/4.0b10