Firefox slow at closing tabs

After upgrading TBB from 2.2.35-9 to 2.2.35-10 (linux i686), Firefox has become significantly slower at closing tabs (both using Ctrl-w or clicking on the tab X icon).

Possible causes: upgrade of Firefox (from 11 to 12), upgrade or other components (see changelog) or a combination of these.

Steps to reproduce: open 4-5 tabs, then try to close them: it will take about 2-3 seconds for each tab to close; with previous versions of the TBB, tabs would close almost instantly.

Trac:
Username: cri