No Btrfs by default in Fedora 16

Random reads are not nearly as much of a problem, as long as the index pages are well cached, which they would be for relatively small files. The problem with most copy on write filesystems is that every random access involves an index lookup to locate the right data page, and the indexes for something that has experienced a lot of random writes have essentially one entry per data page. If you have very large files, that may mean two or more physical I/Os to do a random read, one to look up an uncached index page, and one to read the data page.

It seems to me that what BTRFS needs for something like Firefox is the ability to disable copy-on-write for individual files, except when a user takes a volume snapshot. Alternatively, applications like Firefox could keep a backup database file, and only fsync the primary database file every ten minutes or so, and check on startup to see if the primary database file needs to be recovered from the backup.

I am not sure what you mean. For example mount options will clearly also have an effect, and it seems likely that the performance in this particular benchmark relates to Btrfs defaulting to datacow. Btrfs also supports O(1) file copies so in many ways it has better performance than ext4.

Fixing on disk format is clearly a priority, but that is orthogonal to performance vs. features. For example, checksumming, and the "feature" of allowing hundreds of hardlinks per directory, requires format changes. On the other hand adding a heuristic to toggle datacow probably wouldn't.