We've identified a bug that may cause, under certain circumstances, periodic freezes of the Firefox UI, in terms of a short freeze every about 9 minutes. This only affects a small amount of users, so, if you don't notice this behavior, you can stop reading.
As part of the Memshrink effort, we reduced the SQLite page cache used by Places. This is not a leak, the page cache is actively used by SQLite to improve performances by reducing disk I/O. Originally Places was allowed to use a large cache (maximum 6% of physical memory, per connection), to ensure responsiveness of the awesome bar.