Around Firefox 3.6 the version of SQLite used for these files changed to use 3.7 or greater. And the error you get when you try to use many of the popular SQLite browsers is exactly the one you are seeing. (Error: file is encrypted or is not a database)
I know your working on a *nix box, but if you have access to a windows box, SQLite Expert Personal version 3.4.31.2253 supports browsing this file for Firefox 15.0.1.
The current version of SQLite Browser (which does have a Linux version) does not use SQLite 3.7 so it does not open Firefox 15.01 places.sqlite, or cookies.sqlite..
Sorry I don't have a Linux box handy to test the SQL browsers available there..