Mozilla planning to release Firefox 15.0.1 which supposed to fix Bug 787743 where private browsing mode not working in Firefox 15.0, don’t misinterpret not working of PB mode as unable to start PB mode it rather have to deal with PB feature functionality which has to clear browsing history automatically when you exist from Private browsing. Firefox 15, 16 beta and 17 aurora affected with this bug (Firefox 14 and 18 nightly are not affected), Mozilla planning to offer fix to stable version users worldwide with Firefox 15.0.1 release.
This bug can be always reproducible now in Firefox 15
1. In Firefox, clear history from Firefox button>options>Privacy> click on “clear your recent history”
2. Now visit about:cache page in Firefox and Disk cache device obviously shows zero entries.
3. Now enter private browsing mode, visit couple of sites like Google.com and Gmail, now about:cache in PB shows 4 entries for Disk Cache, you can click on “list cache entries” to view them.
4. Stop private browsing
5. Now in normal mode when you visit about:cache page, Gmail and Google entries were listed on Disk Cache report which shouldn’t happen where Disk cache needs to show 0KB and 0 entries.
If you’re using Firefox 16 beta and Firefox 17 beta, patches are ready to land for them so don’t worry this bug will be fixed in these channels as well.
You can find more discussion here.