After loading a page to about:newtab tab, about:newtab document stays in memory. Looks like it is released after loading some new page to the tab. str: Open FF. Open a new tab (it should have about:newtab loaded) Click some link in the page Run cycle collector, check cc log. about:newtab is there. (bug 728407#c0 can be useful)

If I click a link further about:newtab disappears. See also bug 724239. Is it maybe held alive by the bfcache or something? I thought the bfcache sometimes freezes windows and sometimes throws them away.

Yeah, clicking further makes it disappear. Based on the cc log 0x7fab53fd6740 [rc=6] root nsGenericElement (xhtml) a class='site' about:newtab is the root. IIRC bfcache doesn't have pointers to elements, but to document. Also, bfcached documents are "in CC generation", so CC doesn't traverse them at all.

This could be related to bug 728426, where a document is also kept alive temporarily.